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.

loadtest/internal/edge/encode.go

@@ -37,12 +37,11 @@ func stateReq(gameID string, includeAlphabet bool) []byte {
 	return b.FinishedBytes()
 }
 
-// submitPlay builds a SubmitPlayRequest payload. dir is "H" or "V"; tiles are the
-// newly-placed tiles in main-word order.
-func submitPlay(gameID, dir string, tiles []PlayTile) []byte {
+// submitPlay builds a SubmitPlayRequest payload. tiles are the newly-placed tiles in
+// main-word order; the backend infers the play's orientation from them and the board.
+func submitPlay(gameID string, tiles []PlayTile) []byte {
 	b := flatbuffers.NewBuilder(256)
 	gid := b.CreateString(gameID)
-	d := b.CreateString(dir)
 	offs := make([]flatbuffers.UOffsetT, len(tiles))
 	for i, t := range tiles {
 		fb.PlayTileStart(b)
@@ -59,7 +58,6 @@ func submitPlay(gameID, dir string, tiles []PlayTile) []byte {
 	tilesVec := b.EndVector(len(offs))
 	fb.SubmitPlayRequestStart(b)
 	fb.SubmitPlayRequestAddGameId(b, gid)
-	fb.SubmitPlayRequestAddDir(b, d)
 	fb.SubmitPlayRequestAddTiles(b, tilesVec)
 	b.Finish(fb.SubmitPlayRequestEnd(b))
 	return b.FinishedBytes()