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/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