feat: "multiple words per turn" rule for Russian games

Add a per-game rule chosen on New Game for Russian variants (default off = the
single-word rule; on = standard Scrabble). Off, only the main word along the play
direction is validated and scored; perpendicular cross-words are ignored,
including in robot move generation. The rule rides every create and enqueue
request and joins the matchmaking key, so games and auto-match stay one uniform
path; "Russian-only" is a UI affordance (English always sends standard and shows
no toggle).

- Engine: consume scrabble-solver v1.1.0's PlayOptions{IgnoreCrossWords}, threaded
  through engine.Options.MultipleWordsPerTurn -> playOpts() into validate, score
  and generate.
- Backend: thread the flag through game CreateParams/Game + store (games column),
  lobby InvitationSettings + invitation row, and the matchmaker queue key (variant
  + rule); persisted, so a rebuilt-from-journal game keeps it. Baseline migration
  gains multiple_words_per_turn (DB not versioned); jet regenerated.
- Edge: multiple_words_per_turn added to the EnqueueRequest / CreateInvitationRequest
  FlatBuffers tables (Go + TS regenerated) and threaded through the gateway.
- UI: a "Multiple words per turn" toggle on New Game, shown for Russian variants
  only (auto-match and friend invite), default off; English silently sends standard.
- Tests: backend engine/matchmaker; UI unit (gating) + Playwright e2e (solver
  corner-case + GCG fixtures ship in v1.1.0). Docs + PRERELEASE tracker updated.

ui/src/screens/NewGame.svelte

@@ -7,7 +7,13 @@
   import { navigate } from '../lib/router.svelte';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import type { AccountRef, Variant } from '../lib/model';
-  import { availableVariants, VARIANT_FLAG, VARIANT_RULES } from '../lib/variants';
+  import {
+    availableVariants,
+    VARIANT_FLAG,
+    VARIANT_RULES,
+    supportsMultipleWordsToggle,
+    multipleWordsForRequest,
+  } from '../lib/variants';
 
   // The auto-match move clock (mirrors backend game.DefaultTurnTimeout = 24h).
   const AUTO_MATCH_HOURS = 24;
@@ -15,6 +21,10 @@
   // The offered variants are gated by the languages the sign-in service supports;
   // the auto-match list and the friend-invite picker both use this.
   const variants = $derived(availableVariants(app.session?.supportedLanguages));
+  // "Multiple words per turn" off is the single-word rule; it is offered for Russian games
+  // only (English is always standard and shows no toggle). Shared by both flows.
+  let multipleWords = $state(false);
+  const autoHasRussian = $derived(variants.some((v) => supportsMultipleWordsToggle(v.id)));
   const timeouts = [
     { secs: 300, key: 'time.minutes' as MessageKey, n: 5 },
     { secs: 1800, key: 'time.minutes' as MessageKey, n: 30 },
@@ -70,7 +80,7 @@
     searching = true;
     matched = false;
     try {
-      const r = await gateway.lobbyEnqueue(v);
+      const r = await gateway.lobbyEnqueue(v, multipleWordsForRequest(v, multipleWords));
       if (r.matched && r.game) {
         matched = true;
         searching = false;
@@ -137,6 +147,7 @@
         hintsAllowed: hints > 0,
         hintsPerPlayer: hints,
         dropoutTiles: 'remove',
+        multipleWordsPerTurn: multipleWordsForRequest(inviteVariant, multipleWords),
       });
       showToast(t('new.invited'));
       navigate('/');
@@ -171,6 +182,12 @@
 
       {#if mode === 'auto'}
         <p class="subtitle">{t('new.subtitle')}</p>
+        {#if autoHasRussian}
+          <label class="toggle">
+            <span>{t('new.multipleWordsPerTurn')}</span>
+            <input type="checkbox" bind:checked={multipleWords} />
+          </label>
+        {/if}
         <div class="variants">
           {#each variants as v (v.id)}
             <button class="variant" onclick={() => find(v.id)} disabled={!connection.online}>
@@ -225,6 +242,12 @@
               </select>
             </label>
           </div>
+          {#if inviteVariant && supportsMultipleWordsToggle(inviteVariant)}
+            <label class="toggle">
+              <span>{t('new.multipleWordsPerTurn')}</span>
+              <input type="checkbox" bind:checked={multipleWords} />
+            </label>
+          {/if}
           <button class="invite" disabled={selected.length === 0 || !inviteVariant || !connection.online} onclick={sendInvite}>{t('new.invite')}</button>
         </div>
       {/if}
@@ -385,6 +408,21 @@
   .field select.placeholder {
     color: var(--text-muted);
   }
+  .toggle {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    gap: 8px;
+    padding: 11px;
+    border: 1px solid var(--border);
+    background: var(--surface);
+    border-radius: var(--radius-sm);
+    user-select: none;
+  }
+  .toggle span {
+    font-size: 0.85rem;
+    color: var(--text);
+  }
   .muted {
     color: var(--text-muted);
     margin: 0;