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.

ui/src/lib/i18n/en.ts

@@ -67,7 +67,7 @@ export const en = {
   'game.checkWord': 'Check word',
   'game.dictionary': 'Dictionary',
   'game.dropGame': 'Drop game',
-  'game.preview': 'Scores {n}',
+  'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Not a legal move',
   'game.chooseBlank': 'Choose a letter for the blank',
   'game.exchangeTitle': 'Select tiles to exchange',
@@ -86,7 +86,6 @@ export const en = {
   'game.check': 'Check',
   'game.checkWait': 'Please wait a moment.',
   'game.noHintOptions': 'No options with your letters.',
-  'game.scores': 'Scores: {n}',
   'game.thinking': 'thinking…',
 
   'move.pass': 'pass',