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/components/DebugPanel.svelte

@@ -20,7 +20,7 @@
         const m = await import('../lib/dict');
         const list = await m.listCachedDicts();
         const cached = list.length ? list.map((d) => `${d.key}=${(d.bytes / 1024).toFixed(0)}KB`).join(' ') : 'none';
-        dictInfo = `local eval: ${m.LOCAL_EVAL_ENABLED ? 'on' : 'off'}  breaker: ${m.dictLoadingDisabled() ? 'TRIPPED' : 'ok'}\ndicts cached: ${cached}`;
+        dictInfo = `dict breaker: ${m.dictLoadingDisabled() ? 'TRIPPED' : 'ok'}\ndicts cached: ${cached}`;
       } catch {
         dictInfo = 'local eval: n/a';
       }

ui/src/game/Game.svelte

@@ -260,7 +260,7 @@
     // 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) => {
-        if (m.LOCAL_EVAL_ENABLED) dict = m;
+        dict = m;
       });
     }
   });

ui/src/lib/dict/config.ts

@@ -1,6 +0,0 @@
-// Feature flag for the on-device move preview (the local eval). Default on; append
-// `?nolocal` to the URL to force the network preview instead, for a side-by-side
-// check on the test contour. The network preview is always the fallback, so
-// disabling the flag only removes the acceleration, never the feature.
-export const LOCAL_EVAL_ENABLED =
-  typeof location === 'undefined' || !new URLSearchParams(location.search).has('nolocal');

ui/src/lib/dict/index.ts

@@ -5,4 +5,3 @@
 export { evaluateLocal } from './eval';
 export { getDawg, peekDawg, hasDawg, dictLoadingDisabled, noteDictMiss, clearDictInstances, bustDictHttpCache } from './loader';
 export { clearDictCache, listCachedDicts } from './store';
-export { LOCAL_EVAL_ENABLED } from './config';