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.

pkg/fbs/scrabble.fbs

@@ -151,11 +151,10 @@ table Profile {
 
 // --- game (authenticated) ---
 
-// SubmitPlayRequest places tiles in a direction on the player's turn. tiles are addressed
-// by alphabet index.
+// SubmitPlayRequest places tiles on the player's turn; the backend infers the play's
+// orientation from the tiles and the board. tiles are addressed by alphabet index.
 table SubmitPlayRequest {
   game_id:string;
-  dir:string;
   tiles:[PlayTile];
 }
 
@@ -205,18 +204,19 @@ table ExchangeRequest {
 }
 
 // EvalRequest previews a tentative play without committing it. tiles are addressed by
-// alphabet index.
+// alphabet index; the backend infers the play's orientation from the tiles and the board.
 table EvalRequest {
   game_id:string;
-  dir:string;
   tiles:[PlayTile];
 }
 
-// EvalResult is an unlimited move preview: legality, score and the words formed.
+// EvalResult is an unlimited move preview: legality, score, the words formed (main word
+// first) and the orientation the backend inferred (dir is "H"/"V", empty when illegal).
 table EvalResult {
   legal:bool;
   score:int;
   words:[string];
+  dir:string;
 }
 
 // CheckWordRequest looks a word up in the game's pinned dictionary. word is a sequence of