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).

docs/ARCHITECTURE.md

@@ -352,6 +352,17 @@ Key points:
 - History is dictionary-independent (§9.1): the engine emits decoded
   `MoveRecord`s and reconstructs the board from them with `engine.ReplayBoard`
   (alphabet only, no dictionary).
+- The **client mirrors this validation path for an instant move preview** (the
+  local eval, `ui/src/lib/dict`): the dawg reader and the
+  validate/score/direction slice are ported to TypeScript, byte-for-byte against
+  this engine and pinned by the `conformance` CI job. Composing a move is scored
+  on-device with no round trip. It is an **advisory accelerator only** —
+  `submit_play` re-validates on the server, so a wrong or spoofed local result
+  cannot corrupt a game. The pinned per-game dictionary blob is fetched once
+  through a **session-gated `GET /dict/{variant}/{version}`** edge route
+  (immutable; cached in IndexedDB best-effort) and reused across sessions; any
+  miss, storage eviction or a bad-connection breaker falls back to the network
+  `evaluate`. The warm-up overlay is `docs/UI_DESIGN.md`.
 
 ## 6. Game rules
 

docs/FUNCTIONAL.md

@@ -163,7 +163,10 @@ tile that extends
 an existing word (down a column or across a row) is accepted. A play is validated
 against the game's dictionary at submit time and scored; an unlimited preview
 reports the word(s) a tentative move would form and its score, or that it is not
-legal, and the move is offered for submission only once it is confirmed legal. The dictionary check tool is
+legal, and the move is offered for submission only once it is confirmed legal. The preview is
+computed **on-device** for an instant response once the game's dictionary has loaded — a
+brief warm-up shows on the first open otherwise — and falls back to the server whenever the
+local dictionary is unavailable. The dictionary check tool is
 unlimited and offers a complaint on any result; for a word it finds, it also links out to an
 external reference dictionary (gramota.ru for Russian games, scrabblewordfinder.org for
 English) to look it up. Hints are governed per game —

docs/FUNCTIONAL_ru.md

@@ -171,7 +171,9 @@ _Вход сейчас только через провайдера, поэто
 слово (по столбцу или по строке), принимается. Ход проверяется по словарю партии при
 сдаче и считается; безлимитный предпросмотр показывает слово (или слова), которое
 образует предполагаемый ход, и его очки — либо что ход недопустим, — и ход можно
-отправить только после подтверждения, что он допустим. Инструмент проверки слова безлимитный и
+отправить только после подтверждения, что он допустим. Предпросмотр считается **на устройстве**
+и отвечает мгновенно, как только словарь партии загружен — иначе при первом открытии показывается
+короткий прогрев, — и уходит на сервер, если локальный словарь недоступен. Инструмент проверки слова безлимитный и
 предлагает пожаловаться на любой результат; для найденного слова он также даёт ссылку на
 внешний справочный словарь (gramota.ru для русских игр, scrabblewordfinder.org для английских),
 чтобы его посмотреть. Подсказки управляются настройками

docs/TESTING.md

@@ -21,6 +21,15 @@ tests or touching CI.
   mock for the friends screen (code issue/redeem, accept a request), the lobby
   invitations section, the stats screen, profile editing, and the GCG export's
   finished-only visibility.
+- **Local-eval conformance** — the client's on-device move preview (the ported
+  dawg reader + validator, `ui/src/lib/dict`) is checked byte-for-byte against the
+  authoritative Go engine. `backend/cmd/dictgen` and `backend/cmd/validategen` emit
+  golden vectors from the release dictionaries — every stored word plus a battery of
+  plays across both cross-word rules and all variants, each with the engine's
+  legality, score, words and inferred direction. The gated Vitest suites
+  (`ui/src/lib/dict/*.parity.test.ts`, skipped without their `DICT_*` env) replay
+  them and must agree exactly; the `conformance` CI job runs the whole loop, so a
+  drift in the port fails the merge.
 - **Engine** — correctness of scoring and move generation is owned
   by `scrabble-solver`'s own GCG-backed tests. `backend/internal/engine` adds, on
   top of the embedded solver: per-variant smoke tests (load all three committed

docs/UI_DESIGN.md

@@ -427,6 +427,20 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
   raises a badge on the lobby ⚙️ tab (combined with the friend-request count) and a "1" on the
   Info tab.
 
+## Dictionary warm-up overlay (`components/DictWarmup.svelte`)
+
+Opening a game whose local move-preview dictionary is not yet cached shows a brief,
+non-dismissable warm-up overlay while it downloads (a returning player's cached
+dictionary loads from IndexedDB in a few ms, so the flash-guard suppresses it then).
+A darker-than-onboarding scrim covers
+the board; centred on it, a large emoji cycles through a fixed playful sequence — one
+per 500 ms tick (300 ms still, then a 200 ms clockwise spin with the current glyph
+fading out and the next fading in), looping — under a constant "Loading…" caption.
+Reduce-motion drops the spin to a plain cross-fade. A ~120 ms flash-guard suppresses
+the overlay for a disk-cached dictionary that loads in a few ms; a cold (network) load
+shows it until the dictionary is ready or a 5 s cap elapses, after which the preview
+falls back to the network. See docs/ARCHITECTURE.md §5 (the local eval).
+
 ## Caveat
 
 Emoji are rendered by the platform's system emoji font, so their exact look varies across