feat(rules): forbid repeating a word already on the board in Erudit

Russian "Эрудит" treats a word laid on the board as belonging to the
game: it cannot be laid again. Neither the solver, the backend nor the
offline JS port knew the rule, so a player (and the robot) could replay a
word freely. Official Scrabble places no such restriction, so both
Scrabble variants keep playing unrestricted.

The rule applies in two ways. A play whose main word is already on the
board is illegal, and is neither accepted nor generated. A play whose
perpendicular cross-word is already there stands — that word is
incidental to laying the main word — but scores nothing. The set of
played words is the game's own move journal, main words and cross-words
alike, compared decoded, so a word spelled with a blank is the same word.

It lives in the game layer, not the solver: only a game knows its
history, and the solver stays stateless and standard-rules. The backend
applies it at submit, at the move preview and over generated moves
(filtering and re-ranking them, so neither the robot nor the hint can
offer a play the engine would then refuse); the client port does the same
for the offline engine and for the on-device preview of an online game.

The rule is pinned per game (games.no_repeat_words, set from the variant
at creation) rather than keyed on the variant, because a game is replayed
from its journal on every open. Applied retroactively it would make an
already-played repeat illegal — closing that game as a draw — and would
rescore a play whose cross-word repeats an earlier word, shifting a live
game's totals. Games created before the rule keep playing without it. The
flag rides the wire as a trailing field because the client's preview must
score the way the server does, and offline games pin the same answer in
their own record.

ui/src/lib/localgame/serialize.ts

@@ -35,6 +35,10 @@ export interface LocalGameRecord {
   seed: string;
   players: number;
   multipleWords: boolean;
+  /** Forbids laying a word that is already on the board (the Erudit rule). Pinned at creation from
+   *  the variant, so a game started before the rule existed (where it is absent, read as false)
+   *  replays and scores exactly as it was played. */
+  noRepeatWords?: boolean;
   dropoutTiles: DropoutTiles;
   seats: Seat[];
   /** The dictionary-independent, alphabet-index-space move journal. */
@@ -87,6 +91,7 @@ export function serializeGame(game: LocalGame, meta: RecordMeta): LocalGameRecor
     seed: game.seed.toString(),
     players: game.playerCount,
     multipleWords: cfg.multipleWords,
+    noRepeatWords: cfg.noRepeatWords || undefined,
     dropoutTiles: cfg.dropoutTiles,
     seats: meta.seats,
     journal: game.history,
@@ -113,6 +118,7 @@ export function replayGame(record: LocalGameRecord, dawg: Dawg): LocalGame {
     players: record.players,
     dawg,
     multipleWords: record.multipleWords,
+    noRepeatWords: record.noRepeatWords,
     dropoutTiles: record.dropoutTiles,
   };
   const game = new LocalGame(opts);