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/inttest/game_test.go

@@ -96,11 +96,11 @@ func TestGameLifecycleAndStats(t *testing.T) {
 	for i := 0; i < 300 && !mirror.Over(); i++ {
 		cur := seats[mirror.ToMove()]
 		if hint, ok := mirror.HintView(); ok {
-			last, err = svc.SubmitPlay(ctx, g.ID, cur, hint.Dir, hint.Tiles)
+			last, err = svc.SubmitPlay(ctx, g.ID, cur, hint.Tiles)
 			if err != nil {
 				t.Fatalf("submit play: %v", err)
 			}
-			if _, err := mirror.SubmitPlay(hint.Dir, hint.Tiles); err != nil {
+			if _, err := mirror.SubmitPlay(hint.Tiles); err != nil {
 				t.Fatalf("mirror play: %v", err)
 			}
 		} else {
@@ -154,10 +154,10 @@ func TestReplayEquivalence(t *testing.T) {
 	for i := 0; i < 6 && !mirror.Over(); i++ {
 		cur := seats[mirror.ToMove()]
 		if hint, ok := mirror.HintView(); ok {
-			if _, err := svc.SubmitPlay(ctx, g.ID, cur, hint.Dir, hint.Tiles); err != nil {
+			if _, err := svc.SubmitPlay(ctx, g.ID, cur, hint.Tiles); err != nil {
 				t.Fatalf("submit: %v", err)
 			}
-			mirror.SubmitPlay(hint.Dir, hint.Tiles)
+			mirror.SubmitPlay(hint.Tiles)
 		} else {
 			if _, err := svc.Pass(ctx, g.ID, cur); err != nil {
 				t.Fatalf("pass: %v", err)
@@ -201,7 +201,7 @@ func TestResignWinnerAndStats(t *testing.T) {
 	if !ok {
 		t.Fatal("no opening move")
 	}
-	played, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Dir, hint.Tiles) // p0 scores
+	played, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Tiles) // p0 scores
 	if err != nil {
 		t.Fatalf("p0 play: %v", err)
 	}
@@ -248,7 +248,7 @@ func TestResignOnOpponentTurn(t *testing.T) {
 	if !ok {
 		t.Fatal("no opening move")
 	}
-	if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Dir, hint.Tiles); err != nil { // p0 scores, now p1's turn
+	if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Tiles); err != nil { // p0 scores, now p1's turn
 		t.Fatalf("p0 play: %v", err)
 	}
 
@@ -455,19 +455,22 @@ func TestEvaluatePlayPreview(t *testing.T) {
 	}
 	hint, _ := newMirror(t, seed, 2).HintView()
 
-	eval, err := svc.EvaluatePlay(ctx, g.ID, seats[0], hint.Dir, hint.Tiles)
+	eval, err := svc.EvaluatePlay(ctx, g.ID, seats[0], hint.Tiles)
 	if err != nil {
 		t.Fatalf("evaluate: %v", err)
 	}
 	if !eval.Valid || eval.Score <= 0 {
 		t.Errorf("legal preview = %+v, want valid with score", eval)
 	}
+	if eval.Dir != hint.Dir.String() || len(eval.Words) == 0 {
+		t.Errorf("legal preview dir/words = (%q, %v), want dir %q with the words formed", eval.Dir, eval.Words, hint.Dir.String())
+	}
 	// The same play must still be available afterwards (no commit).
-	if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Dir, hint.Tiles); err != nil {
+	if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Tiles); err != nil {
 		t.Fatalf("submit after evaluate: %v", err)
 	}
 
-	bad, err := svc.EvaluatePlay(ctx, g.ID, seats[1], engine.Horizontal, []engine.TileRecord{{Row: 0, Col: 0, Letter: "q"}})
+	bad, err := svc.EvaluatePlay(ctx, g.ID, seats[1], []engine.TileRecord{{Row: 0, Col: 0, Letter: "q"}})
 	if err != nil {
 		t.Fatalf("evaluate illegal: %v", err)
 	}
@@ -498,7 +501,7 @@ func TestConcurrentSubmitSerialized(t *testing.T) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Dir, hint.Tiles); err == nil {
+			if _, err := svc.SubmitPlay(ctx, g.ID, seats[0], hint.Tiles); err == nil {
 				mu.Lock()
 				ok++
 				mu.Unlock()