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/alphabet.ts

@@ -73,6 +73,12 @@ export function valueForLetter(variant: Variant, letter: string): number {
   return i === undefined ? 0 : t.values[i];
 }
 
+/** alphabetValues returns a variant's tile values indexed by alphabet letter index, or an
+ * empty array when the table is not yet cached. The local move validator scores with it. */
+export function alphabetValues(variant: Variant): readonly number[] {
+  return cache.get(variant)?.values ?? [];
+}
+
 /** indexForLetter maps a display letter to its wire index; a blank ("?") maps to the blank
  * sentinel. It throws when the letter is outside the cached alphabet — a placement bug, not
  * user input (the UI constrains every entry point to the variant's alphabet). */