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.

docs/FUNCTIONAL.md

@@ -92,9 +92,12 @@ settings and the game starts once every invitee has accepted — any decline can
 expires after seven days.
 
 ### Playing a game
-Place tiles, pass, exchange, or resign. A play is validated against the game's
-dictionary at submit time and scored; an unlimited preview reports what a
-tentative move would score and whether it is legal. The dictionary check tool is
+Place tiles, pass, exchange, or resign. Tiles are laid without choosing a
+direction — the game infers the play's orientation, so a single tile that extends
+an existing word (down a column or across a row) is accepted. A play is validated
+against the game's dictionary at submit time and scored; an unlimited preview
+reports the word(s) a tentative move would form and its score, or that it is not
+legal, and the move is offered for submission only once it is confirmed legal. The dictionary check tool is
 unlimited and offers a complaint on any result. Hints are governed per game —
 whether they are allowed and how many each player starts with — and draw on a
 personal hint wallet once the per-game allowance is spent. The game ends when the