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/engine/domain.go

@@ -52,11 +52,25 @@ func fromScrabbleDir(d scrabble.Direction) Direction {
 
 // SubmitPlay validates and applies the current player's play described in decoded
 // terms: each TileRecord carries a concrete letter (the letter a blank stands for
-// when Blank is set) and a board coordinate. It encodes the tiles through the
+// when Blank is set) and a board coordinate. It infers the play's orientation
+// from the tiles and the board (resolveDirection), encodes the tiles through the
 // ruleset alphabet and delegates to Play, so it returns the same errors
 // (ErrTilesNotOnRack, ErrIllegalPlay, ErrGameOver) plus ErrIllegalPlay when a
 // letter is outside the variant's alphabet.
-func (g *Game) SubmitPlay(dir Direction, tiles []TileRecord) (MoveRecord, error) {
+func (g *Game) SubmitPlay(tiles []TileRecord) (MoveRecord, error) {
+	placements, err := g.placements(tiles)
+	if err != nil {
+		return MoveRecord{}, err
+	}
+	return g.Play(resolveDirection(g.board, placements), placements)
+}
+
+// SubmitPlayDir is SubmitPlay with the orientation supplied rather than inferred.
+// It exists for journal replay, which reproduces a committed game exactly from
+// the stored "H"/"V" rather than re-deriving it (docs/ARCHITECTURE.md §9.1):
+// re-derivation would tie historical reconstruction to the current resolver, so
+// replay trusts the recorded direction. Live play uses SubmitPlay.
+func (g *Game) SubmitPlayDir(dir Direction, tiles []TileRecord) (MoveRecord, error) {
 	placements, err := g.placements(tiles)
 	if err != nil {
 		return MoveRecord{}, err
@@ -78,10 +92,11 @@ func (g *Game) SubmitExchange(tiles []string) (MoveRecord, error) {
 
 // EvaluatePlay scores and validates a tentative play without committing it,
 // backing the unlimited "what would my next move score, and is it legal?" tool.
-// It returns the decoded move (placed tiles, the words it forms and its score)
-// or ErrIllegalPlay when the solver rejects it. The board, racks, bag and turn
-// are left untouched.
-func (g *Game) EvaluatePlay(dir Direction, tiles []TileRecord) (MoveRecord, error) {
+// It infers the play's orientation from the tiles and the board exactly as
+// SubmitPlay does, then returns the decoded move (placed tiles, the words it
+// forms, its orientation and its score) or ErrIllegalPlay when the solver
+// rejects it. The board, racks, bag and turn are left untouched.
+func (g *Game) EvaluatePlay(tiles []TileRecord) (MoveRecord, error) {
 	if g.over {
 		return MoveRecord{}, ErrGameOver
 	}
@@ -89,7 +104,7 @@ func (g *Game) EvaluatePlay(dir Direction, tiles []TileRecord) (MoveRecord, erro
 	if err != nil {
 		return MoveRecord{}, err
 	}
-	move, err := g.solver.ValidatePlay(g.board, dir.scrabbleDir(), placements)
+	move, err := g.solver.ValidatePlay(g.board, resolveDirection(g.board, placements), placements)
 	if err != nil {
 		return MoveRecord{}, fmt.Errorf("%w: %v", ErrIllegalPlay, err)
 	}