Backend infers play direction; UI previews words and gates submit on legality

A single tile that only extended a word perpendicular to the client-declared
direction was rejected: the UI always sent dir=H for one-tile plays (the
dirOverride/Controls toggle was orphaned in the Stage 7 game rework), so placing
"А" above "БАК" to form "АБАК" failed the solver's main-word-length check even
though the word is in the dictionary.

Make the backend infer a play's orientation from the placed tiles and the board
(internal/engine.resolveDirection): two or more tiles by the line they share, a
lone tile by the axis it abuts (longer word wins, horizontal on a tie). Direction
becomes an output, not an input: drop dir from the SubmitPlay/Eval wire requests
and add it to EvalResult. Journal replay keeps trusting the stored "H"/"V"
(SubmitPlayDir) so a rebuilt game matches the one committed.

UI: stop computing/sending direction; the preview now shows the words a move
forms with its total score (game.previewWords); the make-move control is disabled
until the play is confirmed legal; the "your turn" label hides while tiles are
pending. Delete the orphaned Controls.svelte.

Regenerate the FlatBuffers bindings (Go + TS) and update the gateway transcode
and the loadtest edge client to the new contract. Bake the decision into
ARCHITECTURE.md (§5/§9.1), FUNCTIONAL.md (+ _ru) and the backend README.

backend/internal/game/service.go

@@ -147,10 +147,12 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 // and, for an exchange, the swapped tiles.
 type engineOp func(g *engine.Game) (engine.MoveRecord, []string, error)
 
-// SubmitPlay validates, scores and commits the player's placement.
-func (svc *Service) SubmitPlay(ctx context.Context, gameID, accountID uuid.UUID, dir engine.Direction, tiles []engine.TileRecord) (MoveResult, error) {
+// SubmitPlay validates, scores and commits the player's placement. The engine
+// infers the play's orientation from the tiles and the board, so the caller
+// supplies only the placed tiles (docs/ARCHITECTURE.md §5).
+func (svc *Service) SubmitPlay(ctx context.Context, gameID, accountID uuid.UUID, tiles []engine.TileRecord) (MoveResult, error) {
 	return svc.transition(ctx, gameID, accountID, func(g *engine.Game) (engine.MoveRecord, []string, error) {
-		rec, err := g.SubmitPlay(dir, tiles)
+		rec, err := g.SubmitPlay(tiles)
 		return rec, nil, err
 	})
 }
@@ -528,7 +530,7 @@ func (svc *Service) timeoutGame(ctx context.Context, gameID uuid.UUID, now time.
 
 // EvaluatePlay previews a tentative play for a seated player against the current
 // board without committing it: whether it is legal and what it would score.
-func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUID, dir engine.Direction, tiles []engine.TileRecord) (EvalResult, error) {
+func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUID, tiles []engine.TileRecord) (EvalResult, error) {
 	pre, err := svc.store.GetGame(ctx, gameID)
 	if err != nil {
 		return EvalResult{}, err
@@ -547,7 +549,7 @@ func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUI
 		return EvalResult{}, err
 	}
 	validateStart := time.Now()
-	rec, err := g.EvaluatePlay(dir, tiles)
+	rec, err := g.EvaluatePlay(tiles)
 	svc.metrics.recordValidate(ctx, pre.Variant, validateStart)
 	if err != nil {
 		if errors.Is(err, engine.ErrIllegalPlay) {
@@ -555,7 +557,7 @@ func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUI
 		}
 		return EvalResult{}, err
 	}
-	return EvalResult{Valid: true, Score: rec.Score, Words: rec.Words}, nil
+	return EvalResult{Valid: true, Score: rec.Score, Words: rec.Words, Dir: rec.Dir.String()}, nil
 }
 
 // CheckWord reports whether word is in the game's pinned dictionary. It is the
@@ -961,7 +963,7 @@ func replayMove(g *engine.Game, mv HistoryMove) error {
 		if mv.Dir == "V" {
 			dir = engine.Vertical
 		}
-		_, err := g.SubmitPlay(dir, mv.Tiles)
+		_, err := g.SubmitPlayDir(dir, mv.Tiles)
 		return err
 	case "pass":
 		_, err := g.Pass()