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/robot/driver.go

@@ -138,7 +138,7 @@ func (s *Service) act(ctx context.Context, rt game.RobotTurn, now time.Time) err
 	var res game.MoveResult
 	switch d.kind {
 	case decidePlay:
-		res, err = s.games.SubmitPlay(ctx, rt.GameID, rt.RobotID, d.move.Dir, d.move.Tiles)
+		res, err = s.games.SubmitPlay(ctx, rt.GameID, rt.RobotID, d.move.Tiles)
 	case decideExchange:
 		res, err = s.games.Exchange(ctx, rt.GameID, rt.RobotID, d.exchange)
 	default: