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/DictWarmup.svelte

@@ -0,0 +1,81 @@
+<script lang="ts">
+  import { onMount, onDestroy } from 'svelte';
+  import { t } from '../lib/i18n/index.svelte';
+  import { app } from '../lib/app.svelte';
+
+  // A cute emoji cycler shown while a game's dictionary warms for the local move
+  // preview (docs/UI_DESIGN.md). One emoji per 500ms tick — 300ms static, then a
+  // 200ms clockwise spin with the current emoji fading out and the next fading in —
+  // looping through the sequence; the ten emojis span the 5s warm-up cap.
+  const EMOJIS = ['🎲', '🤔', '📡', '📀', '💾', '⏳', '🐌', '🙈', '🇷🇺', '✌️'];
+  const TICK_MS = 500;
+  const SPIN_MS = 200;
+
+  let index = $state(0);
+  let timer: ReturnType<typeof setInterval> | null = null;
+
+  onMount(() => {
+    timer = setInterval(() => (index = (index + 1) % EMOJIS.length), TICK_MS);
+  });
+  onDestroy(() => {
+    if (timer) clearInterval(timer);
+  });
+
+  // A simultaneous fade + clockwise spin used as both the in and out transition on the
+  // keyed glyph, so the outgoing and incoming emoji cross-fade in place. Under
+  // reduce-motion it degrades to a plain fade.
+  function spinFade(_node: Element, { duration = SPIN_MS }: { duration?: number } = {}) {
+    const spin = !app.reduceMotion;
+    return {
+      duration,
+      css: (u: number) => (spin ? `opacity:${u};transform:rotate(${(1 - u) * 360}deg)` : `opacity:${u}`),
+    };
+  }
+</script>
+
+<div class="overlay" role="status" aria-live="polite">
+  <div class="box">
+    <div class="stage">
+      {#key index}
+        <span class="emoji" in:spinFade out:spinFade>{EMOJIS[index]}</span>
+      {/key}
+    </div>
+    <span class="caption">{t('dict.loading')}</span>
+  </div>
+</div>
+
+<style>
+  .overlay {
+    position: fixed;
+    inset: 0;
+    z-index: 200;
+    display: grid;
+    place-items: center;
+    /* Darker than the onboarding scrim — this blocks the board until the dictionary is ready. */
+    background: rgba(0, 0, 0, 0.72);
+    padding: env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) env(safe-area-inset-left);
+  }
+  .box {
+    display: grid;
+    justify-items: center;
+    gap: 1rem;
+  }
+  .stage {
+    display: grid;
+    place-items: center;
+    /* Reserve the glyph box so the caption never shifts as the emoji swaps. */
+    width: 4rem;
+    height: 4rem;
+  }
+  .emoji {
+    grid-area: 1 / 1;
+    font-size: 3.25rem; /* about twice a tab-bar button */
+    line-height: 1;
+    will-change: transform, opacity;
+  }
+  .caption {
+    color: #fff;
+    font-size: 1rem;
+    opacity: 0.85;
+  }
+</style>