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/server/handlers_user.go

@@ -26,10 +26,10 @@ func (s *Server) handleProfile(c *gin.Context) {
 	c.JSON(http.StatusOK, profileResponseFor(acc))
 }
 
-// submitPlayRequest places tiles in a direction on the player's turn. Each tile's Letter
-// is a wire alphabet index; for a blank it is the designated letter's index.
+// submitPlayRequest places tiles on the player's turn; the engine infers the
+// play's orientation from the tiles and the board. Each tile's Letter is a wire
+// alphabet index; for a blank it is the designated letter's index.
 type submitPlayRequest struct {
-	Dir   string `json:"dir"`
 	Tiles []struct {
 		Row    int  `json:"row"`
 		Col    int  `json:"col"`
@@ -70,11 +70,6 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 		abortBadRequest(c, "invalid request body")
 		return
 	}
-	dir, ok := parseDirection(req.Dir)
-	if !ok {
-		abortBadRequest(c, "dir must be H or V")
-		return
-	}
 	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
 	if err != nil {
 		s.abortErr(c, err)
@@ -85,7 +80,7 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	res, err := s.games.SubmitPlay(c.Request.Context(), gameID, uid, dir, tiles)
+	res, err := s.games.SubmitPlay(c.Request.Context(), gameID, uid, tiles)
 	if err != nil {
 		s.abortErr(c, err)
 		return