ui/phase-27: battle viewer (radial scene, playback, map markers)

Engine wire change: Report.battle switched from []uuid.UUID to
[]BattleSummary{id, planet, shots} so the map can place battle
markers without N extra fetches. FBS schema + generated Go/TS
regenerated; transcoder + report controller updated; openapi
adds the BattleSummary schema with a freeze test.

Backend gateway forwards engine GET /api/v1/battle/:turn/:uuid as
/api/v1/user/games/{game_id}/battles/{turn}/{battle_id} (handler
plus engineclient.FetchBattle, contract test stub, openapi spec).

UI:
- BattleViewer (lib/battle-player/) is a logically isolated SVG
  radial scene that consumes a BattleReport prop. Planet at the
  centre, races on the outer ring at equal angular spacing, race
  clusters by (race, className) with <class>:<numLeft> labels;
  observer groups (inBattle: false) are not drawn; eliminated
  races drop out and survivors re-distribute on the next frame.
- Shot line per frame: red on destroyed, green otherwise; erased
  on the next frame. Playback controls: play/pause + step ± +
  rewind + 1x/2x/4x speed (400/200/100 ms per frame).
- Page wrapper (lib/active-view/battle.svelte) loads BattleReport
  via api/battle-fetch.ts; synthetic-gameId prefix routes to a
  fixture loader, otherwise REST through the gateway. Always-
  visible <ol> text protocol satisfies the accessibility ask.
- section-battles.svelte links every battle UUID into the viewer.
- map/battle-markers.ts: yellow X cross of 2 LinePrim through the
  corners of the planet's circumscribed square (stroke width
  clamps from 1 px at 1 shot to 5 px at 100+ shots); bombing
  marker is a stroke-only ring (yellow when damaged, red when
  wiped). Wired into state-binding.ts; click handler dispatches
  battle clicks to the viewer and bombing clicks to the matching
  Reports row.
- i18n keys for the viewer in en + ru.

Docs: ui/docs/battle-viewer-ux.md, FUNCTIONAL.md §6.5 + ru
mirror, ui/PLAN.md Phase 27 decisions + deferred TODOs (push
event, richer class visuals, animated re-distribution).

Tests: Vitest unit (radial layout + timeline frame builder +
marker stroke formula + marker primitives), Playwright e2e for
the viewer (Reports link → viewer, playback step, not-found),
backend engineclient FetchBattle (200 / 404 / bad input), engine
openapi freezes (BattleReport, BattleReportGroup,
BattleActionReport, BattleSummary, Report.battle items).

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

backend/openapi.yaml

@@ -1106,6 +1106,44 @@ paths:
           $ref: "#/components/responses/NotImplementedError"
         "500":
           $ref: "#/components/responses/InternalError"
+  /api/v1/user/games/{game_id}/battles/{turn}/{battle_id}:
+    get:
+      tags: [User]
+      operationId: userGamesBattle
+      summary: Read one engine battle report
+      description: |
+        Forwards to the engine's `GET /api/v1/battle/:turn/:uuid`. The
+        engine response body is passed through verbatim. `404 Not Found`
+        is returned when the battle does not exist for the supplied
+        `turn` / `battle_id` pair.
+      security:
+        - UserHeader: []
+      parameters:
+        - $ref: "#/components/parameters/XUserID"
+        - $ref: "#/components/parameters/GameID"
+        - $ref: "#/components/parameters/Turn"
+        - name: battle_id
+          in: path
+          required: true
+          description: Battle identifier (RFC 4122 UUID).
+          schema:
+            type: string
+            format: uuid
+      responses:
+        "200":
+          description: Engine battle report passed through.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/PassthroughObject"
+        "400":
+          $ref: "#/components/responses/InvalidRequestError"
+        "404":
+          $ref: "#/components/responses/NotFoundError"
+        "501":
+          $ref: "#/components/responses/NotImplementedError"
+        "500":
+          $ref: "#/components/responses/InternalError"
   /api/v1/user/sessions:
     get:
       tags: [User]