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.

backend/internal/server/dto.go

@@ -1,8 +1,6 @@
 package server
 
 import (
-	"strings"
-
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
@@ -297,15 +295,3 @@ func chatDTOFrom(m social.Message) chatDTO {
 		CreatedAtUnix: m.CreatedAt.Unix(),
 	}
 }
-
-// parseDirection maps the wire direction string to an engine.Direction.
-func parseDirection(s string) (engine.Direction, bool) {
-	switch strings.ToUpper(strings.TrimSpace(s)) {
-	case "H":
-		return engine.Horizontal, true
-	case "V":
-		return engine.Vertical, true
-	default:
-		return 0, false
-	}
-}

backend/internal/server/dto_test.go

@@ -14,30 +14,6 @@ import (
 	"scrabble/backend/internal/social"
 )
 
-func TestParseDirection(t *testing.T) {
-	cases := map[string]struct {
-		in   string
-		want engine.Direction
-		ok   bool
-	}{
-		"horizontal":      {"H", engine.Horizontal, true},
-		"vertical":        {"V", engine.Vertical, true},
-		"lowercase":       {"h", engine.Horizontal, true},
-		"trimmed":         {" V ", engine.Vertical, true},
-		"invalid":         {"X", 0, false},
-		"empty":           {"", 0, false},
-		"diagonal-is-not": {"D", 0, false},
-	}
-	for name, tc := range cases {
-		t.Run(name, func(t *testing.T) {
-			got, ok := parseDirection(tc.in)
-			if ok != tc.ok || (ok && got != tc.want) {
-				t.Fatalf("parseDirection(%q) = (%v, %v), want (%v, %v)", tc.in, got, ok, tc.want, tc.ok)
-			}
-		})
-	}
-}
-
 func TestStatusForError(t *testing.T) {
 	cases := map[string]struct {
 		err        error

backend/internal/server/handlers_game.go

@@ -22,11 +22,14 @@ type hintResultDTO struct {
 	HintsRemaining int           `json:"hints_remaining"`
 }
 
-// evalResultDTO is an unlimited move preview: legality, score and the words formed.
+// evalResultDTO is an unlimited move preview: legality, score, the words formed
+// (main word first) and the orientation the engine inferred ("H"/"V", empty when
+// illegal).
 type evalResultDTO struct {
 	Legal bool     `json:"legal"`
 	Score int      `json:"score"`
 	Words []string `json:"words"`
+	Dir   string   `json:"dir"`
 }
 
 // wordCheckDTO is the result of the unlimited dictionary lookup tool.
@@ -186,11 +189,6 @@ func (s *Server) handleEvaluate(c *gin.Context) {
 		abortBadRequest(c, "invalid request body")
 		return
 	}
-	dir, ok := parseDirection(req.Dir)
-	if !ok {
-		abortBadRequest(c, "dir must be H or V")
-		return
-	}
 	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
 	if err != nil {
 		s.abortErr(c, err)
@@ -201,12 +199,12 @@ func (s *Server) handleEvaluate(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	ev, err := s.games.EvaluatePlay(c.Request.Context(), gameID, uid, dir, tiles)
+	ev, err := s.games.EvaluatePlay(c.Request.Context(), gameID, uid, tiles)
 	if err != nil {
 		s.abortErr(c, err)
 		return
 	}
-	c.JSON(http.StatusOK, evalResultDTO{Legal: ev.Valid, Score: ev.Score, Words: ev.Words})
+	c.JSON(http.StatusOK, evalResultDTO{Legal: ev.Valid, Score: ev.Score, Words: ev.Words, Dir: ev.Dir})
 }
 
 // handleCheckWord looks a word up in the game's pinned dictionary. The word arrives as

backend/internal/server/handlers_test.go

@@ -75,18 +75,9 @@ func TestRateLimitReportEndpoint(t *testing.T) {
 	}
 }
 
-func TestSubmitPlayRejectsBadDirection(t *testing.T) {
-	headers := map[string]string{"X-User-ID": uuid.New().String()}
-	path := "/api/v1/user/games/" + uuid.New().String() + "/play"
-	rec := do(t, newRoutingServer(), http.MethodPost, path, `{"dir":"X","tiles":[]}`, headers)
-	if rec.Code != http.StatusBadRequest {
-		t.Fatalf("submit play bad dir = %d, want 400", rec.Code)
-	}
-}
-
 func TestSubmitPlayRejectsBadGameID(t *testing.T) {
 	headers := map[string]string{"X-User-ID": uuid.New().String()}
-	rec := do(t, newRoutingServer(), http.MethodPost, "/api/v1/user/games/not-a-uuid/play", `{"dir":"H"}`, headers)
+	rec := do(t, newRoutingServer(), http.MethodPost, "/api/v1/user/games/not-a-uuid/play", `{}`, headers)
 	if rec.Code != http.StatusBadRequest {
 		t.Fatalf("submit play bad game id = %d, want 400", rec.Code)
 	}

backend/internal/server/handlers_user.go

@@ -26,10 +26,10 @@ func (s *Server) handleProfile(c *gin.Context) {
 	c.JSON(http.StatusOK, profileResponseFor(acc))
 }
 
-// submitPlayRequest places tiles in a direction on the player's turn. Each tile's Letter
-// is a wire alphabet index; for a blank it is the designated letter's index.
+// submitPlayRequest places tiles on the player's turn; the engine infers the
+// play's orientation from the tiles and the board. Each tile's Letter is a wire
+// alphabet index; for a blank it is the designated letter's index.
 type submitPlayRequest struct {
-	Dir   string `json:"dir"`
 	Tiles []struct {
 		Row    int  `json:"row"`
 		Col    int  `json:"col"`
@@ -70,11 +70,6 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 		abortBadRequest(c, "invalid request body")
 		return
 	}
-	dir, ok := parseDirection(req.Dir)
-	if !ok {
-		abortBadRequest(c, "dir must be H or V")
-		return
-	}
 	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
 	if err != nil {
 		s.abortErr(c, err)
@@ -85,7 +80,7 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	res, err := s.games.SubmitPlay(c.Request.Context(), gameID, uid, dir, tiles)
+	res, err := s.games.SubmitPlay(c.Request.Context(), gameID, uid, tiles)
 	if err != nil {
 		s.abortErr(c, err)
 		return