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/ARCHITECTURE.md

@@ -262,6 +262,13 @@ Key points:
   check for resign. The engine exposes a
   decoded, solver-free API (`SubmitPlay`/`SubmitExchange`/`EvaluatePlay`/
   `HintView`/`Hand`) so `internal/game` drives it without importing the solver.
+  A play's **orientation (H/V) is inferred from the placed tiles and the board**,
+  not supplied by the caller: two or more tiles fix it by the line they share; a
+  lone tile takes the axis along which it abuts existing tiles (the longer word
+  winning, horizontal on a tie), so a single tile extending an existing word
+  vertically is accepted. Journal replay instead trusts the **stored** direction
+  (`SubmitPlayDir`, §9.1) to reproduce a committed game exactly rather than
+  re-deriving it.
 - The **game domain** (`internal/game`) owns everything the engine does not —
   persistence, turn scheduling, the configurable turn timeout / auto-resign, the
   hint budget, word-check complaints, history and GCG — and is the engine's only
@@ -469,7 +476,8 @@ and — in a per-move JSON payload — the acting player's rack before the move
 `?` for a blank), and for a play its direction, main-word anchor, placed tiles
 (letter as text, coordinate, blank flag) and the words formed; for an exchange,
 the swapped tiles. This is exactly what is needed both to **replay the game
-through the engine** (a cache miss) and to render history or emit GCG **without a
+through the engine** (a cache miss; replay trusts the stored direction rather than
+re-deriving it, so the rebuild matches the committed game) and to render history or emit GCG **without a
 dictionary**: the board for visual replay is reconstructed by applying placements
 onto an empty grid, since moves were validated at play time and scores are
 stored. `variant` and `dict_version` are kept as **metadata only** (audit,