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/lib/premiums.ts

@@ -85,5 +85,19 @@ export function centre(variant: Variant): { row: number; col: number } {
   return { row: 7, col: 7 };
 }
 
-// Tile values and the per-variant alphabet now arrive from the server (lib/alphabet.ts);
-// the board geometry above is all this module owns.
+/** RACK_SIZE is the number of tiles drawn to a full rack — 7 for every variant. It sizes
+ *  the all-tiles (bingo) bonus test in the local move preview. */
+export const RACK_SIZE = 7;
+
+/** BINGO is the all-tiles bonus per variant, ported from scrabble-solver/rules/rules.go:
+ *  50 for the Scrabble variants, 15 for Эрудит. The local move preview adds it when a play
+ *  uses the whole rack. The dict conformance test pins these to the engine's rulesets. */
+export const BINGO: Record<Variant, number> = {
+  scrabble_en: 50,
+  scrabble_ru: 50,
+  erudit_ru: 15,
+};
+
+// Tile values and the per-variant alphabet arrive from the server (lib/alphabet.ts); the
+// board geometry, centre, rack size and bingo above are the static per-variant constants
+// this module owns (all ported from scrabble-solver/rules/rules.go).