feat: on-device move preview (local eval) with network fallback

Score and validate a tentative move on-device instead of a per-arrangement network
round trip. The dawg reader and the validate/score/direction slice of the
scrabble-solver engine are ported to TypeScript (ui/src/lib/dict), pinned
byte-for-byte to the Go engine by a `conformance` CI job (full-dictionary reader
parity plus a battery of plays across every variant and both cross-word rules,
including the inferred orientation). The server stays authoritative — submit_play
re-validates — so the local result is an advisory accelerator only.

- backend: Registry.DictBytes + an authed GET /api/v1/user/dict/{variant}/{version}
  (immutable) streaming the pinned per-game dawg; caddy routes /dict to the gateway.
- gateway: a session-gated /dict edge route proxying it; fetchDict on the transport.
- client: the dictionary loads on game open (low priority so it never starves the
  game on a slow link; aborted at a 5s cap or when leaving the game), is cached in
  IndexedDB (best-effort, self-healing on a rejected blob) and reused across
  sessions; a warm-up overlay covers a cold load, then the network preview is the
  fallback; a bad-connection breaker stops warming after repeated misses; the move
  preview cancels its in-flight request when the tiles change.
- parity generators backend/cmd/{dictgen,validategen} + gated Vitest suites, run in
  CI against the release dictionaries. A hidden debug readout lists the cached
  dictionaries + breaker state, and its reset clears the cache.
- docs: ARCHITECTURE §5, TESTING, UI_DESIGN, FUNCTIONAL (+ru).

ui/src/game/Game.svelte

@@ -4,6 +4,7 @@
   import TabBar from '../components/TabBar.svelte';
   import TapConfirm from '../components/TapConfirm.svelte';
   import Modal from '../components/Modal.svelte';
+  import DictWarmup from '../components/DictWarmup.svelte';
   import Board from './Board.svelte';
   import Rack from './Rack.svelte';
   import { gateway } from '../lib/gateway';
@@ -50,6 +51,15 @@
   let placement = $state<Placement>(newPlacement([]));
   let preview = $state<EvalResult | null>(null);
   let busy = $state(false);
+
+  // The local move-preview subsystem (dynamically imported when enabled) and the warm-up
+  // overlay state. dict is null until the subsystem loads, or when the feature is disabled.
+  let dict = $state<typeof import('../lib/dict') | null>(null);
+  let dictWarming = $state(false);
+  let warmStarted = false;
+  // Aborts the in-flight dictionary download (at the warm-up cap, or when leaving the game)
+  // so a large download does not keep starving the channel on a slow link.
+  let warmCtrl: AbortController | null = null;
   let zoomed = $state(false);
   let selected = $state<number | null>(null);
   let focus = $state<{ row: number; col: number } | null>(null);
@@ -244,6 +254,24 @@
     void load();
     void loadFriends();
     void loadBlocked();
+    // Load the local move-preview subsystem (kept out of the initial bundle) when enabled,
+    // so composing a move can be scored on-device; the network preview stays the fallback.
+    // Skipped in mock mode (no dictionary server; the mock uses the network preview path and
+    // must never see the warm-up overlay, which would intercept the e2e's taps).
+    if (import.meta.env.MODE !== 'mock') {
+      void import('../lib/dict').then((m) => {
+        dict = m;
+      });
+    }
+  });
+
+  // Warm the game's dictionary for the local move preview once, when both the game and the
+  // dynamically-imported preview subsystem are available (see warmDict).
+  $effect(() => {
+    if (dict && view && !warmStarted) {
+      warmStarted = true;
+      void warmDict();
+    }
   });
 
   // cacheSnapshot returns the open game's current state as a CachedGame for the delta reducers.
@@ -569,6 +597,10 @@
     clearReorder();
   }
   onDestroy(() => {
+    // Leaving the game: abort a still-downloading dictionary and any in-flight preview so they
+    // stop holding the channel.
+    warmCtrl?.abort();
+    evalCtrl?.abort();
     window.removeEventListener('pointermove', onWinMove);
     window.removeEventListener('pointerup', onWinUp);
     window.removeEventListener('pointerdown', onExtraPointer);
@@ -638,19 +670,74 @@
     scheduleDraftSave();
   }
 
+  // warmDict loads the game's dictionary for the local move preview when the player opens the
+  // game. When it is already cached the preview is instant and no overlay shows; a cold
+  // dictionary downloads behind a non-dismissable warm-up overlay, which hides on load or after
+  // a 5s cap — the preview then uses the network until the download finishes in the background.
+  // The bad-connection breaker skips the overlay and goes straight to the network.
+  async function warmDict() {
+    const d = dict;
+    const v = view;
+    if (!d || !v) return;
+    if (d.hasDawg(v.game.variant, v.game.dictVersion) || d.dictLoadingDisabled()) return;
+    const ctrl = new AbortController();
+    warmCtrl = ctrl;
+    let settled = false;
+    // A short flash-guard: a disk-cached dictionary loads in a few ms, so only show the
+    // overlay if the load has not settled by now — a cold (network) load always outlasts it.
+    const grace = setTimeout(() => {
+      if (!settled) dictWarming = true;
+    }, 120);
+    const loaded = await Promise.race([
+      d.getDawg(v.game.variant, v.game.dictVersion, ctrl.signal),
+      new Promise<null>((r) => setTimeout(() => r(null), 5000)),
+    ]);
+    settled = true;
+    clearTimeout(grace);
+    dictWarming = false;
+    if (!loaded) {
+      // Did not load within the cap: abort the download so it stops starving the channel this
+      // session, and count the miss toward the bad-connection breaker (3 -> stop warming).
+      ctrl.abort();
+      d.noteDictMiss();
+    }
+  }
+
   let previewTimer: ReturnType<typeof setTimeout> | null = null;
+  let evalCtrl: AbortController | null = null;
   function recompute() {
     preview = null;
     if (previewTimer) clearTimeout(previewTimer);
+    // The tiles changed: cancel any in-flight network preview (evaluate is non-mutating, so
+    // aborting is safe) — a stale, out-of-order response cannot overwrite the newer one, and
+    // rapid placements do not pile up requests on a slow link.
+    evalCtrl?.abort();
+    evalCtrl = null;
     // Off-turn the composition is position-only: no score preview or evaluate.
     if (!isMyTurn) return;
     const sub = toSubmit(placement);
     if (!sub) return;
+    // Instant on-device preview when the game's dictionary is warm; the network otherwise.
+    const d = dict;
+    const v = view;
+    if (d && v) {
+      const reader = d.peekDawg(v.game.variant, v.game.dictVersion);
+      if (reader && hasAlphabet(v.game.variant)) {
+        try {
+          preview = d.evaluateLocal(reader, v.game.variant, board, sub.tiles, v.game.multipleWordsPerTurn);
+          return;
+        } catch {
+          /* fall through to the network preview */
+        }
+      }
+    }
+    const ctrl = new AbortController();
+    evalCtrl = ctrl;
     previewTimer = setTimeout(async () => {
       try {
-        preview = await gateway.evaluate(id, sub.tiles, variant);
+        preview = await gateway.evaluate(id, sub.tiles, variant, ctrl.signal);
       } catch {
-        /* best-effort */
+        /* best-effort (or aborted) */
       }
     }, 250);
   }
@@ -1122,6 +1209,11 @@
   {/if}
 </Screen>
 
+<!-- Warm-up overlay while the game's dictionary loads for the local move preview. -->
+{#if dictWarming}
+  <DictWarmup />
+{/if}
+
 <!-- Reusable game-screen pieces, arranged differently by the portrait and landscape branches
      above so the markup and logic stay single-sourced (see the {#if landscape} split). -->
 {#snippet scoreboardBlock()}