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.

pkg/fbs/scrabblefb/GameView.go

@@ -221,8 +221,20 @@ func (rcv *GameView) MutateKind(n int32) bool {
 	return rcv._tab.MutateInt32Slot(34, n)
 }
 
+func (rcv *GameView) NoRepeatWords() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(36))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *GameView) MutateNoRepeatWords(n bool) bool {
+	return rcv._tab.MutateBoolSlot(36, n)
+}
+
 func GameViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(16)
+	builder.StartObject(17)
 }
 func GameViewAddId(builder *flatbuffers.Builder, id flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(id), 0)
@@ -275,6 +287,9 @@ func GameViewAddUnreadMessages(builder *flatbuffers.Builder, unreadMessages bool
 func GameViewAddKind(builder *flatbuffers.Builder, kind int32) {
 	builder.PrependInt32Slot(15, kind, 0)
 }
+func GameViewAddNoRepeatWords(builder *flatbuffers.Builder, noRepeatWords bool) {
+	builder.PrependBoolSlot(16, noRepeatWords, false)
+}
 func GameViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }