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/gen/fbs/scrabblefb/eval-request.ts

@@ -30,37 +30,26 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-dir():string|null
-dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
-dir(optionalEncoding?:any):string|Uint8Array|null {
-  const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
-}
-
 tiles(index: number, obj?:PlayTile):PlayTile|null {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
 static startEvalRequest(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
-static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(1, dirOffset, 0);
-}
-
 static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(2, tilesOffset, 0);
+  builder.addFieldOffset(1, tilesOffset, 0);
 }
 
 static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
@@ -80,10 +69,9 @@ static endEvalRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   return offset;
 }
 
-static createEvalRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createEvalRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
   EvalRequest.startEvalRequest(builder);
   EvalRequest.addGameId(builder, gameIdOffset);
-  EvalRequest.addDir(builder, dirOffset);
   EvalRequest.addTiles(builder, tilesOffset);
   return EvalRequest.endEvalRequest(builder);
 }

ui/src/gen/fbs/scrabblefb/eval-result.ts

@@ -42,8 +42,15 @@ wordsLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+dir():string|null
+dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+dir(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 10);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
 static startEvalResult(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(4);
 }
 
 static addLegal(builder:flatbuffers.Builder, legal:boolean) {
@@ -70,16 +77,21 @@ static startWordsVector(builder:flatbuffers.Builder, numElems:number) {
   builder.startVector(4, numElems, 4);
 }
 
+static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(3, dirOffset, 0);
+}
+
 static endEvalResult(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createEvalResult(builder:flatbuffers.Builder, legal:boolean, score:number, wordsOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createEvalResult(builder:flatbuffers.Builder, legal:boolean, score:number, wordsOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset):flatbuffers.Offset {
   EvalResult.startEvalResult(builder);
   EvalResult.addLegal(builder, legal);
   EvalResult.addScore(builder, score);
   EvalResult.addWords(builder, wordsOffset);
+  EvalResult.addDir(builder, dirOffset);
   return EvalResult.endEvalResult(builder);
 }
 }

ui/src/gen/fbs/scrabblefb/submit-play-request.ts

@@ -30,37 +30,26 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-dir():string|null
-dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
-dir(optionalEncoding?:any):string|Uint8Array|null {
-  const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
-}
-
 tiles(index: number, obj?:PlayTile):PlayTile|null {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
 static startSubmitPlayRequest(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
-static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(1, dirOffset, 0);
-}
-
 static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(2, tilesOffset, 0);
+  builder.addFieldOffset(1, tilesOffset, 0);
 }
 
 static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
@@ -80,10 +69,9 @@ static endSubmitPlayRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   return offset;
 }
 
-static createSubmitPlayRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createSubmitPlayRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
   SubmitPlayRequest.startSubmitPlayRequest(builder);
   SubmitPlayRequest.addGameId(builder, gameIdOffset);
-  SubmitPlayRequest.addDir(builder, dirOffset);
   SubmitPlayRequest.addTiles(builder, tilesOffset);
   return SubmitPlayRequest.endSubmitPlayRequest(builder);
 }