ui/phase-27: mass-based circles + cloud cluster + height fit

Three Phase-27 BattleViewer refinements on top of the radial scene:

1. Height fit. The viewer is pinned to `calc(100dvh − 80px)` so it
   never pushes the in-game shell past the viewport. `.active-view`
   gains `overflow: hidden` + flex column; `.viewer` becomes a
   `flex: 1` child; the always-visible text log shrinks to a 30 dvh
   ceiling with its own scroll. A global `body { margin: 0 }`
   reset (added to `app.html`) plugs the 16 px the browser's
   default body margin used to leak.

2. Mass-based ship-class circles. New `lib/battle-player/mass.ts`
   carries the radius formula and the per-battle FullMass compute:
   `MIN_RADIUS + (MAX_RADIUS − MIN_RADIUS) * sqrt(mass / max)`,
   clamped to `[6, 24] px`. FullMass goes through the existing
   wasm bridge (`emptyMass` → `carryingMass` → `fullMass`) — no
   new wire fields. The viewer page resolves a
   `(race, className) → ShipClassRef` lookup from the parent
   GameReport's `localShipClass` + `otherShipClass` tables and
   passes it to the viewer via context. Unknown class or
   degenerate (weapons/armament) params fall back to MAX_RADIUS
   so the bucket stays visible.

3. Cloud cluster layout. Cluster key shifts from per-group
   `g.key` to `(raceId, className)` so tech-variants of the same
   hull collapse into one visual bucket. The horizontal
   classCircleX row is replaced by a Vogel sunflower spiral in
   the local `(u, v)` basis — `u` points from the race anchor to
   the planet, `v` is `u` rotated 90° clockwise. Buckets are
   sorted by NumberLeft desc; the cluster anchor is pushed inward
   by a quarter step so rank-0 sits closest to the planet. The
   step is adaptive (`min(baseStep, MAX_CLUSTER_RADIUS / sqrt(N))`)
   so clusters with many classes do not spill into neighbours.

Tests:
- Vitest: `radiusForMass` covering zero / max / quarter-mass /
  out-of-range cases (6 cases).
- Playwright: new `battle-viewer.spec.ts` case asserts
  `document.documentElement.scrollHeight - window.innerHeight ≤ 4`
  at a 1280×720 desktop viewport. The existing fixture gains
  `localShipClass` + `otherShipClass` so the lookup has data to
  render proportional circles.

Docs: `ui/docs/battle-viewer-ux.md` rewrites the "Radial scene"
section (cloud layout, mass-based radius, height fit) and adds
a "Height fit" subsection. `docs/FUNCTIONAL.md` §6.5 (+ ru
mirror) get the one-line story about per-mass sizing, cluster
aggregation, and the viewport-locked layout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/lib/active-view/battle.svelte

@@ -2,11 +2,16 @@
 Phase 27 — active-view wrapper around the BattleViewer. Loads the
 BattleReport for the supplied `gameId`/`turn`/`battleId` and either
 shows the radial playback (BattleViewer), a loading skeleton, or a
-not-found state. The viewer itself is a logically isolated
-component that takes a `BattleReport` prop — this wrapper owns
-loading and routing concerns.
+not-found state.
+
+This wrapper also bridges the surrounding GameReport's ship-class
+tables into a `(race, className) → ShipClassRef` lookup the viewer
+needs to size class circles by ship mass. The viewer remains
+prop-driven; we just resolve the lookup once here so the lower
+component does not have to know about `RenderedReportSource`.
 -->
 <script lang="ts">
+	import { getContext } from "svelte";
 	import { goto } from "$app/navigation";
 
 	import {
@@ -15,6 +20,15 @@ loading and routing concerns.
 		type BattleReport,
 	} from "../../api/battle-fetch";
 	import { i18n } from "$lib/i18n/index.svelte";
+	import {
+		RENDERED_REPORT_CONTEXT_KEY,
+		type RenderedReportSource,
+	} from "$lib/rendered-report.svelte";
+	import {
+		MapShipClassLookup,
+		type ShipClassLookup,
+		type ShipClassRef,
+	} from "../battle-player/mass";
 
 	import BattleViewer from "../battle-player/battle-viewer.svelte";
 
@@ -28,6 +42,42 @@ loading and routing concerns.
 		battleId: string;
 	} = $props();
 
+	const rendered = getContext<RenderedReportSource | undefined>(
+		RENDERED_REPORT_CONTEXT_KEY,
+	);
+
+	// shipClassLookup turns `(race, className)` into the five class
+	// parameters required by `calc.EmptyMass`. Local classes belong
+	// to the report recipient (`report.race`); foreign classes carry
+	// their own `race` field. Lookup is cheap to rebuild whenever the
+	// report changes — the active-view-host re-renders on turn flips
+	// anyway.
+	const shipClassLookup = $derived.by<ShipClassLookup>(() => {
+		const map = new Map<string, ShipClassRef>();
+		const report = rendered?.report;
+		if (report) {
+			for (const cls of report.localShipClass) {
+				map.set(`${report.race}::${cls.name}`, {
+					drive: cls.drive,
+					weapons: cls.weapons,
+					armament: cls.armament,
+					shields: cls.shields,
+					cargo: cls.cargo,
+				});
+			}
+			for (const cls of report.otherShipClass) {
+				map.set(`${cls.race}::${cls.name}`, {
+					drive: cls.drive,
+					weapons: cls.weapons,
+					armament: cls.armament,
+					shields: cls.shields,
+					cargo: cls.cargo,
+				});
+			}
+		}
+		return new MapShipClassLookup(map);
+	});
+
 	let state = $state<
 		| { kind: "loading" }
 		| { kind: "ready"; report: BattleReport }
@@ -86,7 +136,7 @@ loading and routing concerns.
 			{i18n.t("game.battle.loading")}
 		</p>
 	{:else if state.kind === "ready"}
-		<BattleViewer report={state.report} />
+		<BattleViewer report={state.report} shipClassLookup={shipClassLookup} />
 	{:else if state.kind === "not_found"}
 		<p class="status" data-testid="battle-not-found">
 			{i18n.t("game.battle.not_found")}
@@ -98,7 +148,26 @@ loading and routing concerns.
 
 <style>
 	.active-view {
+		display: flex;
+		flex-direction: column;
+		/*
+		 * The in-game shell renders this active view inside an
+		 * `.active-view-host` with `flex: 1; overflow-y: auto`, but
+		 * the surrounding `.game-shell` uses `min-height: 100vh`,
+		 * so without a hard upper bound the viewer pushes the
+		 * whole shell past the viewport. We pin the active view to
+		 * `100dvh` minus a small allowance for the header chrome
+		 * (in-game Header + optional HistoryBanner = ~66 px on
+		 * desktop) so the internal flex chain can split the
+		 * remaining height between the scene and the always-
+		 * visible log without forcing a page-level scroll.
+		 */
+		height: calc(100dvh - 80px);
+		max-height: calc(100dvh - 80px);
+		min-height: 0;
+		overflow: hidden;
 		padding: 1rem;
+		box-sizing: border-box;
 		font-family: system-ui, sans-serif;
 		color: #d6dcf2;
 	}
@@ -107,6 +176,7 @@ loading and routing concerns.
 		gap: 0.5rem;
 		max-width: 880px;
 		margin: 0 auto 1rem;
+		flex: 0 0 auto;
 	}
 	.back-btn {
 		appearance: none;

ui/frontend/src/lib/battle-player/battle-scene.svelte

@@ -2,16 +2,33 @@
 BattleScene — radial SVG visualisation of one battle frame.
 
 Layout: planet at the centre, race anchors equally spaced on an
-outer ring, each race rendered as a cluster of small class circles
-labelled `<className>:<numLeft>` underneath. The shot line for the
-current frame's `lastAction` is drawn from attacker group to
-defender group; red when the shot destroyed the defender, green
-otherwise. Observer groups (`inBattle === false`) are filtered out
-by `buildFrames`, so they never appear here.
+outer ring, each race rendered as a *cloud* of class circles
+arranged on a Vogel sunflower spiral biased toward the planet.
+Tech-variant groups of the same (race, className) collapse to one
+visual node — the per-tech detail lives in Reports. Each circle's
+radius scales with the per-ship FullMass (sqrt) so heavy ships
+visually dominate.
+
+Observer groups (`inBattle === false`) are filtered out by
+`buildFrames`, so they never appear here. Same-race opponents are
+forbidden by the engine's combat filter, so a shot can never
+collapse to a single visual node.
 -->
 <script lang="ts">
+	import { getContext } from "svelte";
+
 	import type { BattleReport } from "../../api/battle-fetch";
+	import {
+		CORE_CONTEXT_KEY,
+		type CoreHandle,
+	} from "$lib/core-context.svelte";
 	import { layoutRaces } from "./radial-layout";
+	import {
+		computeBattleGroupMass,
+		radiusForMass,
+		MAX_RADIUS,
+		type ShipClassLookup,
+	} from "./mass";
 	import {
 		buildGroupRaceMap,
 		normaliseGroups,
@@ -21,50 +38,113 @@ by `buildFrames`, so they never appear here.
 	let {
 		report,
 		frame,
+		shipClassLookup,
 	}: {
 		report: BattleReport;
 		frame: Frame;
+		shipClassLookup?: ShipClassLookup;
 	} = $props();
 
 	const VIEW_BOX = 800;
 	const CENTER = { x: VIEW_BOX / 2, y: VIEW_BOX / 2 };
 	const PLANET_RADIUS = 60;
 	const RACE_RING_RADIUS = 280;
-	const CLASS_CIRCLE_RADIUS = 24;
-	const CLASS_SPACING = 64;
+	// Vogel sunflower step + half-circle bias toward planet.
+	const BASE_STEP = 1.8 * MAX_RADIUS;
+	const GOLDEN_ANGLE = Math.PI * (3 - Math.sqrt(5));
+	const ANCHOR_BIAS = 0.25; // fraction of step pushed toward planet
+	const MAX_CLUSTER_RADIUS = 0.6 * (RACE_RING_RADIUS - PLANET_RADIUS);
+
+	const coreHandle = getContext<CoreHandle | undefined>(CORE_CONTEXT_KEY);
 
 	const groupRace = $derived(buildGroupRaceMap(report.protocol));
 	const allGroups = $derived(normaliseGroups(report));
 
 	type ClusterEntry = {
-		key: number;
+		bucketKey: string;
 		className: string;
+		race: string;
+		raceId: number;
+		groupKeys: number[];
 		numLeft: number;
+		mass: number;
+		radius: number;
 	};
 
+	// Aggregate every `(raceId, className)` into a single bucket and
+	// compute per-bucket NumberLeft (sum across tech-variants) and
+	// per-ship FullMass via the wasm bridge. mass=0 when the class
+	// either doesn't resolve in the lookup or the calc rejects its
+	// params; downstream `radiusForMass` falls back to MAX_RADIUS for
+	// those nodes.
 	const clustersByRace = $derived.by(() => {
+		const core = coreHandle?.core ?? null;
+		// First pass: build the bucket list per race.
 		const out = new Map<number, ClusterEntry[]>();
+		const bucketIndex = new Map<string, ClusterEntry>();
 		for (const g of allGroups) {
-			const numLeft = frame.remaining.get(g.key) ?? 0;
-			const list = out.get(g.raceId) ?? [];
-			list.push({
-				key: g.key,
-				className: g.group.className,
-				numLeft,
-			});
-			out.set(g.raceId, list);
+			const bucketKey = `${g.raceId}::${g.group.className}`;
+			let bucket = bucketIndex.get(bucketKey);
+			if (bucket === undefined) {
+				const classDef =
+					shipClassLookup?.get(g.group.race, g.group.className) ?? null;
+				const mass = core
+					? computeBattleGroupMass(g.group, classDef, core)
+					: 0;
+				bucket = {
+					bucketKey,
+					className: g.group.className,
+					race: g.group.race,
+					raceId: g.raceId,
+					groupKeys: [],
+					numLeft: 0,
+					mass,
+					radius: MAX_RADIUS,
+				};
+				bucketIndex.set(bucketKey, bucket);
+				const list = out.get(g.raceId) ?? [];
+				list.push(bucket);
+				out.set(g.raceId, list);
+			}
+			bucket.groupKeys.push(g.key);
+			bucket.numLeft += frame.remaining.get(g.key) ?? 0;
 		}
-		// Stable cluster order: by classname then key.
+
+		// Per-battle mass normalisation: the heaviest visual node
+		// renders at MAX_RADIUS; lighter ones scale by sqrt(m/max).
+		let maxMass = 0;
+		for (const bucket of bucketIndex.values()) {
+			if (bucket.mass > maxMass) maxMass = bucket.mass;
+		}
+		for (const bucket of bucketIndex.values()) {
+			bucket.radius = radiusForMass(bucket.mass, maxMass);
+		}
+
+		// Sort buckets in each cluster by NumberLeft desc → rank 0 is
+		// the biggest group (will be placed closest to planet).
 		for (const list of out.values()) {
 			list.sort((a, b) => {
-				const byName = a.className.localeCompare(b.className);
-				if (byName !== 0) return byName;
-				return a.key - b.key;
+				if (b.numLeft !== a.numLeft) return b.numLeft - a.numLeft;
+				return a.className.localeCompare(b.className);
 			});
 		}
 		return out;
 	});
 
+	// bucketByGroupKey lets shot endpoints look up the aggregated
+	// node by any of its constituent ship-group keys.
+	const bucketByGroupKey = $derived.by(() => {
+		const out = new Map<number, ClusterEntry>();
+		for (const list of clustersByRace.values()) {
+			for (const bucket of list) {
+				for (const key of bucket.groupKeys) {
+					out.set(key, bucket);
+				}
+			}
+		}
+		return out;
+	});
+
 	const raceLayout = $derived(
 		layoutRaces(frame.activeRaceIds, {
 			center: CENTER,
@@ -72,23 +152,62 @@ by `buildFrames`, so they never appear here.
 		}),
 	);
 
-	function classCircleX(index: number, count: number): number {
-		const span = (count - 1) * CLASS_SPACING;
-		return -span / 2 + index * CLASS_SPACING;
+	type ClusterBasis = {
+		anchorX: number;
+		anchorY: number;
+		ux: number;
+		uy: number;
+		vx: number;
+		vy: number;
+		step: number;
+	};
+
+	// clusterBasisById carries the local frame for every active
+	// race's cluster: `u` points from the race anchor toward the
+	// planet, `v` is `u` rotated 90° clockwise, and `step` is the
+	// adaptive Vogel-spiral step (scaled down when many class
+	// buckets share a cluster to stay inside MAX_CLUSTER_RADIUS).
+	const clusterBasisById = $derived.by(() => {
+		const out = new Map<number, ClusterBasis>();
+		for (const anchor of raceLayout) {
+			const dx = CENTER.x - anchor.x;
+			const dy = CENTER.y - anchor.y;
+			const len = Math.hypot(dx, dy) || 1;
+			const ux = dx / len;
+			const uy = dy / len;
+			const vx = uy; // (ux, uy) rotated 90° clockwise → (uy, -ux);
+			const vy = -ux; // use mirror to keep label below cluster.
+			const count = (clustersByRace.get(anchor.raceId) ?? []).length;
+			const baseStep = BASE_STEP;
+			const denom = Math.max(1, Math.sqrt(Math.max(count, 1)));
+			const step = Math.min(baseStep, MAX_CLUSTER_RADIUS / denom);
+			const anchorX = anchor.x + ANCHOR_BIAS * step * ux;
+			const anchorY = anchor.y + ANCHOR_BIAS * step * uy;
+			out.set(anchor.raceId, { anchorX, anchorY, ux, uy, vx, vy, step });
+		}
+		return out;
+	});
+
+	function nodePosition(basis: ClusterBasis, rank: number) {
+		const radius = basis.step * Math.sqrt(rank);
+		const angle = rank * GOLDEN_ANGLE;
+		const offsetU = radius * Math.cos(angle);
+		const offsetV = radius * Math.sin(angle);
+		return {
+			x: basis.anchorX + offsetU * basis.ux + offsetV * basis.vx,
+			y: basis.anchorY + offsetU * basis.uy + offsetV * basis.vy,
+		};
 	}
 
 	function findClassCircleCenter(groupKey: number) {
-		const raceId = groupRace.get(groupKey);
-		if (raceId === undefined) return null;
-		const anchor = raceLayout.find((a) => a.raceId === raceId);
-		if (!anchor) return null;
-		const cluster = clustersByRace.get(raceId) ?? [];
-		const idx = cluster.findIndex((c) => c.key === groupKey);
-		if (idx === -1) return null;
-		return {
-			x: anchor.x + classCircleX(idx, cluster.length),
-			y: anchor.y,
-		};
+		const bucket = bucketByGroupKey.get(groupKey);
+		if (!bucket) return null;
+		const basis = clusterBasisById.get(bucket.raceId);
+		if (!basis) return null;
+		const cluster = clustersByRace.get(bucket.raceId) ?? [];
+		const rank = cluster.indexOf(bucket);
+		if (rank === -1) return null;
+		return nodePosition(basis, rank);
 	}
 
 	const shotLine = $derived.by(() => {
@@ -105,6 +224,13 @@ by `buildFrames`, so they never appear here.
 		for (const g of allGroups) {
 			out.set(g.raceId, g.group.race);
 		}
+		// `groupRace` covers protocol-derived races even when no group
+		// is left to read from `allGroups` (shouldn't happen with
+		// `inBattle: true` rosters, but keeps the label resolver
+		// defensive).
+		for (const [, raceId] of groupRace) {
+			if (!out.has(raceId)) out.set(raceId, `race ${raceId}`);
+		}
 		return out;
 	});
 </script>
@@ -112,6 +238,7 @@ by `buildFrames`, so they never appear here.
 <svg
 	class="battle-scene"
 	viewBox="0 0 {VIEW_BOX} {VIEW_BOX}"
+	preserveAspectRatio="xMidYMid meet"
 	role="img"
 	aria-label="battle scene"
 	data-testid="battle-scene"
@@ -132,6 +259,7 @@ by `buildFrames`, so they never appear here.
 
 	{#each raceLayout as anchor (anchor.raceId)}
 		{@const cluster = clustersByRace.get(anchor.raceId) ?? []}
+		{@const basis = clusterBasisById.get(anchor.raceId)}
 		<g
 			class="race-cluster"
 			data-testid="battle-race-cluster"
@@ -139,30 +267,29 @@ by `buildFrames`, so they never appear here.
 		>
 			<text
 				x={anchor.x}
-				y={anchor.y - CLASS_CIRCLE_RADIUS - 12}
+				y={anchor.y - MAX_RADIUS - 12}
 				text-anchor="middle"
 				class="race-label"
 			>{raceLabelById.get(anchor.raceId) ?? `race ${anchor.raceId}`}</text>
-			{#each cluster as entry, i (entry.key)}
-				{@const cx = anchor.x + classCircleX(i, cluster.length)}
-				<g
-					class="class-marker"
-					data-testid="battle-class-marker"
-					data-group-key={entry.key}
-				>
-					<circle
-						cx={cx}
-						cy={anchor.y}
-						r={CLASS_CIRCLE_RADIUS}
-					/>
-					<text
-						x={cx}
-						y={anchor.y + CLASS_CIRCLE_RADIUS + 16}
-						text-anchor="middle"
-						class="class-label"
-					>{entry.className}:{entry.numLeft}</text>
-				</g>
-			{/each}
+			{#if basis}
+				{#each cluster as entry, rank (entry.bucketKey)}
+					{@const pos = nodePosition(basis, rank)}
+					<g
+						class="class-marker"
+						data-testid="battle-class-marker"
+						data-bucket-key={entry.bucketKey}
+						data-class-name={entry.className}
+					>
+						<circle cx={pos.x} cy={pos.y} r={entry.radius} />
+						<text
+							x={pos.x}
+							y={pos.y + entry.radius + 12}
+							text-anchor="middle"
+							class="class-label"
+						>{entry.className}:{entry.numLeft}</text>
+					</g>
+				{/each}
+			{/if}
 		</g>
 	{/each}
 
@@ -183,7 +310,7 @@ by `buildFrames`, so they never appear here.
 <style>
 	.battle-scene {
 		width: 100%;
-		height: auto;
+		height: 100%;
 		background: #0a0d1a;
 		display: block;
 	}
@@ -210,7 +337,7 @@ by `buildFrames`, so they never appear here.
 	}
 	.class-label {
 		fill: #b8c0e6;
-		font-size: 12px;
+		font-size: 11px;
 		font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
 	}
 	.shot {

ui/frontend/src/lib/battle-player/battle-viewer.svelte

@@ -10,12 +10,19 @@ is logically isolated: feed it any `BattleReport` matching
 
 	import { i18n } from "$lib/i18n/index.svelte";
 	import type { BattleReport } from "../../api/battle-fetch";
+	import type { ShipClassLookup } from "./mass";
 
 	import BattleScene from "./battle-scene.svelte";
 	import PlaybackControls from "./playback-controls.svelte";
 	import { buildFrames } from "./timeline";
 
-	let { report }: { report: BattleReport } = $props();
+	let {
+		report,
+		shipClassLookup,
+	}: {
+		report: BattleReport;
+		shipClassLookup?: ShipClassLookup;
+	} = $props();
 
 	const frames = $derived(buildFrames(report));
 	let frameIndex = $state(0);
@@ -81,7 +88,7 @@ is logically isolated: feed it any `BattleReport` matching
 	</header>
 
 	<div class="scene">
-		<BattleScene {report} {frame} />
+		<BattleScene {report} {frame} {shipClassLookup} />
 	</div>
 
 	<PlaybackControls
@@ -113,6 +120,9 @@ is logically isolated: feed it any `BattleReport` matching
 		flex-direction: column;
 		gap: 0.75rem;
 		max-width: 880px;
+		width: 100%;
+		flex: 1 1 auto;
+		min-height: 0;
 		margin: 0 auto;
 		padding: 1rem;
 		color: #d6dcf2;
@@ -122,6 +132,7 @@ is logically isolated: feed it any `BattleReport` matching
 		display: flex;
 		justify-content: space-between;
 		align-items: baseline;
+		flex: 0 0 auto;
 	}
 	.header h2 {
 		margin: 0;
@@ -139,6 +150,16 @@ is logically isolated: feed it any `BattleReport` matching
 		border: 1px solid #1e264a;
 		border-radius: 4px;
 		overflow: hidden;
+		flex: 1 1 auto;
+		min-height: 0;
+	}
+	.log {
+		flex: 0 1 auto;
+		min-height: 4rem;
+		max-height: 30vh;
+		overflow-y: auto;
+		display: flex;
+		flex-direction: column;
 	}
 	.log h3 {
 		margin: 0 0 0.4rem;
@@ -146,15 +167,17 @@ is logically isolated: feed it any `BattleReport` matching
 		font-size: 0.8rem;
 		text-transform: uppercase;
 		letter-spacing: 0.05em;
+		flex: 0 0 auto;
 	}
 	.log ol {
 		list-style: decimal inside;
 		margin: 0;
 		padding: 0;
 		font-size: 0.85rem;
-		max-height: 14rem;
 		overflow-y: auto;
 		color: #c6cdf0;
+		flex: 1 1 auto;
+		min-height: 0;
 	}
 	.log li {
 		padding: 0.15rem 0;

ui/frontend/src/lib/battle-player/mass.ts

@@ -0,0 +1,109 @@
+// Mass helpers for the Battle Viewer.
+//
+// Phase 27 refinement: ship-class circles are sized by per-ship
+// FullMass (Empty + carrying), with a 6..24 px range. The viewer
+// resolves a `(race, className) → ShipClassRef` lookup from the
+// surrounding GameReport's `localShipClass` / `otherShipClass`
+// tables and feeds it through the wasm bridge to `pkg/calc/ship.go`.
+//
+// Pure utilities live here; the Svelte components consume them.
+
+import type { Core } from "../../platform/core/index";
+import type { BattleReportGroup } from "../../api/battle-fetch";
+
+/** Smallest visible ship circle. Picked so the `<class>:<n>` label
+ * stays legible on every viewport. */
+export const MIN_RADIUS = 6;
+
+/** Largest ship circle. Matches the Phase-27 baseline so heavy
+ * ships keep their previous visual prominence. */
+export const MAX_RADIUS = 24;
+
+/**
+ * ShipClassRef is the minimum slice of a ship class needed to
+ * compute its mass. Mirrors the relevant fields of
+ * `ShipClassSummary` (own classes) and `ReportOtherShipClass`
+ * (foreign classes) without coupling the viewer to either type.
+ */
+export interface ShipClassRef {
+	drive: number;
+	weapons: number;
+	armament: number;
+	shields: number;
+	cargo: number;
+}
+
+/**
+ * ShipClassLookup resolves `(race, className)` to a ship-class
+ * descriptor. Returns `null` when the class is not in the parent
+ * report — happens with legacy-mode foreign races that lack a
+ * `<Race> Ship Types` block.
+ */
+export interface ShipClassLookup {
+	get(race: string, className: string): ShipClassRef | null;
+}
+
+/**
+ * computeBattleGroupMass returns the per-ship FullMass for a given
+ * battle group. Mass=0 means "unknown" — either the wasm bridge
+ * rejected the ship-class params (degenerate weapons/armament pair)
+ * or the class did not resolve in the lookup. Either way the
+ * caller's downstream `radiusForMass` falls back to MAX_RADIUS so
+ * the node stays visible.
+ *
+ * Cargo never changes during a battle, so this can be cached per
+ * `(race, className)` bucket for the lifetime of the viewer
+ * session.
+ */
+export function computeBattleGroupMass(
+	group: BattleReportGroup,
+	classDef: ShipClassRef | null,
+	core: Core,
+): number {
+	if (classDef === null) return 0;
+	const empty = core.emptyMass({
+		drive: classDef.drive,
+		weapons: classDef.weapons,
+		armament: classDef.armament,
+		shields: classDef.shields,
+		cargo: classDef.cargo,
+	});
+	if (empty === null) return 0;
+	const cargoTech = classDef.cargo * (group.tech.CARGO ?? 0);
+	const carrying = core.carryingMass({
+		load: group.loadQuantity,
+		cargoTech,
+	});
+	return core.fullMass({ emptyMass: empty, carryingMass: carrying });
+}
+
+/**
+ * radiusForMass maps an absolute ship mass to a circle radius via
+ * a per-battle normalisation: the heaviest visual node always
+ * renders at MAX_RADIUS, lighter ones scale by sqrt(mass /
+ * maxMassInBattle) so the smallest ships don't disappear and the
+ * heaviest ones don't dominate the scene at >MAX_RADIUS. mass<=0
+ * falls back to MAX_RADIUS so unresolved/invalid classes stay
+ * visible.
+ */
+export function radiusForMass(mass: number, maxMassInBattle: number): number {
+	if (maxMassInBattle <= 0 || mass <= 0) return MAX_RADIUS;
+	const scaled = MIN_RADIUS + (MAX_RADIUS - MIN_RADIUS) * Math.sqrt(mass / maxMassInBattle);
+	if (scaled < MIN_RADIUS) return MIN_RADIUS;
+	if (scaled > MAX_RADIUS) return MAX_RADIUS;
+	return scaled;
+}
+
+/**
+ * MapShipClassLookup is a `Map<string, ShipClassRef>`-backed
+ * implementation of `ShipClassLookup`. Key encoding mirrors the
+ * one battle.svelte uses when populating the lookup from the
+ * parent GameReport.
+ */
+export class MapShipClassLookup implements ShipClassLookup {
+	constructor(private readonly map: Map<string, ShipClassRef>) {}
+
+	get(race: string, className: string): ShipClassRef | null {
+		return this.map.get(`${race}::${className}`) ?? null;
+	}
+}