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_test.go

@@ -25,7 +25,7 @@ func TestSubmitPlayMatchesHint(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	rec, err := g.SubmitPlay(hint.Dir, hint.Tiles)
+	rec, err := g.SubmitPlay(hint.Tiles)
 	if err != nil {
 		t.Fatalf("submit play: %v", err)
 	}
@@ -85,7 +85,7 @@ func TestEvaluatePlayDoesNotCommit(t *testing.T) {
 	boardBefore := g.BoardClone()
 	scoreBefore, toMoveBefore, bagBefore := g.Score(0), g.ToMove(), g.BagLen()
 
-	rec, err := g.EvaluatePlay(hint.Dir, hint.Tiles)
+	rec, err := g.EvaluatePlay(hint.Tiles)
 	if err != nil {
 		t.Fatalf("evaluate play: %v", err)
 	}
@@ -106,7 +106,7 @@ func TestEvaluatePlayDoesNotCommit(t *testing.T) {
 func TestEvaluatePlayRejectsIllegal(t *testing.T) {
 	g := newEnglishGame(t, 1)
 	letter := g.Hand(0)[0]
-	_, err := g.EvaluatePlay(Horizontal, []TileRecord{{Row: 0, Col: 0, Letter: letter}})
+	_, err := g.EvaluatePlay([]TileRecord{{Row: 0, Col: 0, Letter: letter}})
 	if !errors.Is(err, ErrIllegalPlay) {
 		t.Errorf("evaluate off-centre opening = %v, want ErrIllegalPlay", err)
 	}