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.

docs/ARCHITECTURE.md

@@ -282,6 +282,17 @@ Key points:
 
 - **Word legality: validate-at-submit.** An illegal play is rejected by
   `Solver.ValidatePlay`; there is no challenge phase.
+- **Multiple words per turn (Russian games).** Russian variants carry a per-game
+  **single-word rule**, chosen on New Game (default **off** = single word; 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; on, every
+  cross-word must be a real word and is scored. The engine threads it as
+  `scrabble.PlayOptions{IgnoreCrossWords}` (solver `v1.1.0`); connectivity and the
+  first-move centre rule are unaffected. The "Russian-only" limit is a **UI affordance**:
+  the backend and engine are variant-agnostic about the flag, and English games always send
+  it on (standard). For auto-match the rule is part of the matchmaking key, so only players
+  who chose the same rule are paired (the rule field rides every create/enqueue request, so
+  matchmaking stays one uniform path).
 - **End of game**: the bag is empty **and** a player empties their rack, **or**
   **6 consecutive scoreless turns** (passes/exchanges), **or** a resignation, or
   a missed turn. The **per-game turn timeout** is chosen at creation

docs/FUNCTIONAL.md

@@ -85,7 +85,12 @@ unrestricted). Variants are shown by their **display name** — both Scrabble va
 the same name titles the in-game screen. This gates only **starting** a new game — both auto-match and a friend
 invitation — so a player still sees and plays existing games of any language. Auto-match
 (always 2 players) joins a per-variant pool and is paired with the next waiting human;
-after 10 s with no human the robot substitutes. Friend games (2–4) are
+after 10 s with no human the robot substitutes. For Russian games (auto-match or friend
+invitation), New Game also offers **"Multiple words per turn"** (default **off**): off plays
+the simplified **single-word rule** — only the word laid along the player's line must be a
+real word, and any incidental perpendicular words are ignored and not scored — while on is
+standard Scrabble. English games are always standard and show no such toggle. In auto-match
+the choice joins the pairing key, so a player only meets opponents who picked the same rule. Friend games (2–4) are
 formed by inviting players from the friend list (an invitation, like a friend code,
 is shareable as a Telegram deep link that opens it directly): the inviter chooses the
 settings and the game starts once every invitee has accepted — any decline cancels it, and an unanswered invitation

docs/FUNCTIONAL_ru.md

@@ -89,7 +89,13 @@ nudge) приходят от бота **этой партии** — по язы
 приглашение друга, — поэтому игрок по-прежнему видит и играет существующие игры на
 любом языке. Авто-подбор (всегда 2 игрока)
 встаёт в пул по варианту и сводится со следующим ожидающим человеком; через 10 с
-без человека подставляется робот. Игры с друзьями (2–4)
+без человека подставляется робот. Для русских игр (авто-подбор или приглашение) на экране
+новой игры есть опция **«Несколько слов за ход»** (по умолчанию **выключена**): выключена —
+упрощённое **правило одного слова**: настоящим словом должно быть только слово, выложенное
+вдоль линии хода, а случайные перпендикулярные слова игнорируются и не засчитываются;
+включена — обычный скрэббл. Английские игры всегда по стандартным правилам и тоггл не
+показывают. В авто-подборе выбор входит в ключ подбора, поэтому игрок сводится только с теми,
+кто выбрал то же правило. Игры с друзьями (2–4)
 формируются приглашением игроков из списка друзей (приглашение, как и код друга,
 можно отправить deep-link'ом в Telegram, который откроет его сразу): инициатор
 выбирает настройки, и партия стартует, когда приняли все приглашённые — любой отказ отменяет приглашение, а без

docs/UI_DESIGN.md

@@ -182,7 +182,9 @@ IV 🏅; active games show Your move 🟢 / Opponent's move ⏳; invitations use
 - **Invitations**: a lobby **section** (a 💌 row per open invitation) with Accept /
   Decline for an invitee and a waiting/Cancel state for the inviter; creating one is the
   **"Play with friends"** mode in `NewGame.svelte` (pick invitees, then variant / move
-  time / hints).
+  time / hints). For a **Russian** variant (auto-match or invite) a **"Multiple words per
+  turn"** checkbox (`.toggle`, **default off** = the single-word rule) appears; English
+  variants never show it.
 - **Statistics** (`screens/Stats.svelte`, the lobby 📊 tab): a 2-column grid of stat
   cards (wins / losses / draws / games / win-rate / best game / best move) — pure
   numbers, no charts.