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/resign_test.go

@@ -12,7 +12,7 @@ func TestResignLeadingPlayerStillLoses(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	played, err := g.SubmitPlay(hint.Dir, hint.Tiles)
+	played, err := g.SubmitPlay(hint.Tiles)
 	if err != nil {
 		t.Fatalf("player 0 play: %v", err)
 	}
@@ -56,7 +56,7 @@ func TestResignTrailingPlayerLoses(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	if _, err := g.SubmitPlay(hint.Dir, hint.Tiles); err != nil { // player 0 scores
+	if _, err := g.SubmitPlay(hint.Tiles); err != nil { // player 0 scores
 		t.Fatalf("player 0 play: %v", err)
 	}
 
@@ -79,7 +79,7 @@ func TestResignSeatOffTurn(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	if _, err := g.SubmitPlay(hint.Dir, hint.Tiles); err != nil { // player 0 moves
+	if _, err := g.SubmitPlay(hint.Tiles); err != nil { // player 0 moves
 		t.Fatalf("player 0 play: %v", err)
 	}
 	if g.ToMove() != 1 {
@@ -165,7 +165,7 @@ func TestMultiplayerLastActiveWins(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	played, err := g.SubmitPlay(hint.Dir, hint.Tiles) // seat 0 takes the lead
+	played, err := g.SubmitPlay(hint.Tiles) // seat 0 takes the lead
 	if err != nil {
 		t.Fatalf("seat 0 play: %v", err)
 	}
@@ -245,7 +245,7 @@ func TestResignedSeatExcludedFromWinOnScorelessEnd(t *testing.T) {
 	if !ok {
 		t.Fatal("opening game has no hint")
 	}
-	played, err := g.SubmitPlay(hint.Dir, hint.Tiles) // seat 0 leads
+	played, err := g.SubmitPlay(hint.Tiles) // seat 0 leads
 	if err != nil {
 		t.Fatalf("seat 0 play: %v", err)
 	}