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.

gateway/internal/backendclient/api.go

@@ -225,9 +225,9 @@ func (c *Client) Profile(ctx context.Context, userID string) (ProfileResp, error
 
 // SubmitPlay commits a placement on the player's turn. The tiles are addressed by alphabet
 // index.
-func (c *Client) SubmitPlay(ctx context.Context, userID, gameID, dir string, tiles []PlayTileJSON) (MoveResultResp, error) {
+func (c *Client) SubmitPlay(ctx context.Context, userID, gameID string, tiles []PlayTileJSON) (MoveResultResp, error) {
 	var out MoveResultResp
-	body := map[string]any{"dir": dir, "tiles": tiles}
+	body := map[string]any{"tiles": tiles}
 	err := c.do(ctx, http.MethodPost, "/api/v1/user/games/"+url.PathEscape(gameID)+"/play", userID, "", body, &out)
 	return out, err
 }
@@ -279,11 +279,13 @@ type HintResultResp struct {
 	HintsRemaining int            `json:"hints_remaining"`
 }
 
-// EvalResultResp is an unlimited move preview.
+// EvalResultResp is an unlimited move preview. Dir is the orientation the backend
+// inferred ("H"/"V", empty when illegal); Words lists the words formed, main word first.
 type EvalResultResp struct {
 	Legal bool     `json:"legal"`
 	Score int      `json:"score"`
 	Words []string `json:"words"`
+	Dir   string   `json:"dir"`
 }
 
 // WordCheckResp is a dictionary lookup outcome.
@@ -365,10 +367,10 @@ func (c *Client) HideGame(ctx context.Context, userID, gameID string) error {
 
 // Evaluate previews a tentative play's legality and score. The tiles are addressed by
 // alphabet index.
-func (c *Client) Evaluate(ctx context.Context, userID, gameID, dir string, tiles []PlayTileJSON) (EvalResultResp, error) {
+func (c *Client) Evaluate(ctx context.Context, userID, gameID string, tiles []PlayTileJSON) (EvalResultResp, error) {
 	var out EvalResultResp
 	err := c.do(ctx, http.MethodPost, c.gamePath(gameID, "/evaluate"), userID, "",
-		map[string]any{"dir": dir, "tiles": tiles}, &out)
+		map[string]any{"tiles": tiles}, &out)
 	return out, err
 }