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()

loadtest/internal/edge/ops.go

@@ -27,8 +27,8 @@ func (c *Client) History(ctx context.Context, token, gameID string) ([]Move, str
 }
 
 // SubmitPlay commits a play and returns the post-move game.
-func (c *Client) SubmitPlay(ctx context.Context, token, gameID, dir string, tiles []PlayTile) (Game, string, error) {
-	r, err := c.execute(ctx, token, msgSubmitPlay, submitPlay(gameID, dir, tiles))
+func (c *Client) SubmitPlay(ctx context.Context, token, gameID string, tiles []PlayTile) (Game, string, error) {
+	r, err := c.execute(ctx, token, msgSubmitPlay, submitPlay(gameID, tiles))
 	if err != nil || r.Code != "ok" {
 		return Game{}, r.Code, err
 	}

loadtest/internal/moves/moves.go

@@ -78,11 +78,10 @@ func (r *Registry) Close() {
 	}
 }
 
-// Action is a chosen turn. Kind is "play", "exchange" or "pass". A play carries Dir
-// ("H"/"V") and Tiles; an exchange carries Exchange (rack indices to swap).
+// Action is a chosen turn. Kind is "play", "exchange" or "pass". A play carries Tiles
+// (the backend infers the orientation); an exchange carries Exchange (rack indices to swap).
 type Action struct {
 	Kind     string
-	Dir      string
 	Tiles    []edge.PlayTile
 	Exchange []byte
 }
@@ -106,7 +105,7 @@ func (r *Registry) Pick(variant string, history []edge.Move, rackIdx []byte, bag
 		return noPlay(rackIdx, bagLen >= e.rs.RackSize), nil
 	}
 	m := midRanked(legal, rng)
-	return Action{Kind: "play", Dir: dirString(m.Dir), Tiles: toPlayTiles(m.Tiles)}, nil
+	return Action{Kind: "play", Tiles: toPlayTiles(m.Tiles)}, nil
 }
 
 // toPlayTiles maps the solver's newly-placed tiles to the edge submit-play tiles
@@ -176,11 +175,3 @@ func noPlay(rackIdx []byte, canExchange bool) Action {
 	}
 	return Action{Kind: "pass"}
 }
-
-// dirString renders a solver direction as the "H"/"V" the edge submit-play expects.
-func dirString(d scrabble.Direction) string {
-	if d == scrabble.Vertical {
-		return "V"
-	}
-	return "H"
-}

loadtest/internal/moves/moves_test.go

@@ -146,9 +146,6 @@ func TestPickWithDawg(t *testing.T) {
 		if len(act.Tiles) == 0 {
 			t.Error("play action has no tiles")
 		}
-		if act.Dir != "H" && act.Dir != "V" {
-			t.Errorf("dir = %q, want H or V", act.Dir)
-		}
 	case "exchange", "pass":
 		// acceptable when the rack has no legal first move
 	default:

loadtest/internal/scenario/scenario.go

@@ -197,7 +197,7 @@ func (d *Driver) playTurn(ctx context.Context, c *edge.Client, p seed.Account, g
 	switch action.Kind {
 	case "play":
 		t0 = time.Now()
-		_, code, _ := c.SubmitPlay(ctx, p.Token, g.ID, action.Dir, action.Tiles)
+		_, code, _ := c.SubmitPlay(ctx, p.Token, g.ID, action.Tiles)
 		d.rec.Record("game.submit_play", code, time.Since(t0))
 	case "exchange":
 		t0 = time.Now()