feat(server): layout-aware room.create + resolved layout echoes (Phase C)

Extends the WebSocket protocol so clients can request a specific
starting layout when creating a room, and receive the resolved
layout echoed back on room.created / room.joined.

Protocol additions (Zod):
- room.create.payload.layout: optional discriminated union of
  { kind: "premade", id } | { kind: "fen", fen, name? }
                           | { kind: "custom", pieces, name? }
- room.created / room.joined: new optional 'layout' field with
  resolved { id, name, pieces } — present on new servers, absent
  on legacy ones (backward compat).
- LAYOUT_INVALID error code for validation failures.
- PiecePlacement + ResolvedLayout schemas.

Server implementation:
- New layouts.ts: resolveLayoutRequest() handles premade lookup,
  FEN parse, custom pieces. Chess960 picks a random seed server-
  side (the ultimate authority on 'which position'). Always runs
  validateLayout — invalid input returns LAYOUT_INVALID, never
  mutates room state.
- rooms.ts: Room.layout field + RoomRegistry.createRoom/joinRoom
  accept/return resolved layouts. Default is CLASSIC_LAYOUT.
- game-session.ts: GameSession + GameSessionRegistry.create
  accept StartingLayout; constructs ChessEngine({ layout }).
- broadcast.ts: handleRoomCreate resolves+validates first, rejects
  with LAYOUT_INVALID toast, otherwise threads layout into the
  room + session + response. handleRoomJoin echoes the room's
  stored layout to the joiner.

Chess package:
- packages/chess/src/index.ts exports LAYOUT_REGISTRY, all premade
  layouts, buildChess960Layout, toFen/fromFen, validateLayout,
  and the related types — so server can pull them without
  importing internal module paths.

Tests: 20 new tests (11 in protocol.test.ts for the layout union,
12 in layouts.test.ts for server-side resolution).

1011 tests passing; bun run check clean.

PROTOCOL.md updated with examples of all three layout kinds.

packages/server/PROTOCOL.md

@@ -54,11 +54,23 @@ Request payload:
 
 ```json
 {
-  "rulesetIds": ["pawns-move-backward", "piece-hp"]
+  "rulesetIds": ["pawns-move-backward", "piece-hp"],
+  "layout": { "kind": "premade", "id": "dunsany" }
 }
 ```
 
 - `rulesetIds`: optional array of preset rule IDs to activate for this game
+- `layout`: optional starting-layout selector. Discriminated union:
+  - `{ "kind": "premade", "id": "dunsany" }` — look up by registered id
+    (`classic`, `dunsany`, `monster`, `pawns-only`, `horde`,
+    `knightmate`, `chess960`, `empty`).
+  - `{ "kind": "fen", "fen": "rnbqkbnr/...", "name": "optional" }` — parse
+    the piece-placement field of a FEN string. Extra FEN fields
+    (side-to-move, castling, en-passant, clocks) are ignored.
+  - `{ "kind": "custom", "pieces": [...], "name": "optional" }` — direct
+    placements; each piece is `{ type, color, square, hasMoved? }`.
+    `square` is 0..63. Capped at 128 pieces.
+- When `layout` is omitted, the server uses the Classic (FIDE) layout.
 
 Response (Server → Client, type `room.created`):
 
@@ -69,14 +81,26 @@ Response (Server → Client, type `room.created`):
   "payload": {
     "code": "ABC123",
     "token": "550e8400-e29b-41d4-a716-446655440000",
-    "color": "white"
+    "color": "white",
+    "layout": {
+      "id": "dunsany",
+      "name": "Dunsany's Chess",
+      "pieces": [{ "type": "pawn", "color": "white", "square": 0 }, ...]
+    }
   }
 }
 ```
 
+- `layout`: resolved starting layout echoed back. Present on all
+  `room.created` and `room.joined` responses from new servers; may be
+  absent on legacy (pre-layouts) servers.
+
 Error cases:
 
 - Server at capacity (too many rooms): `error` code `SERVER_FULL`
+- Layout fails validation (bad king count, duplicate squares,
+  unknown premade id, malformed FEN): `error` code `LAYOUT_INVALID`.
+  Message field carries the human-readable reason.
 
 ### Message: room.join
 
@@ -101,11 +125,19 @@ Response (Server → Client, type `room.joined`):
     "code": "ABC123",
     "token": "661f9500-f30c-52e5-b827-557766550111",
     "color": "black",
-    "activeRules": ["pawns-move-backward"]
+    "activeRules": ["pawns-move-backward"],
+    "layout": {
+      "id": "dunsany",
+      "name": "Dunsany's Chess",
+      "pieces": [{ "type": "pawn", "color": "white", "square": 0 }, ...]
+    }
   }
 }
 ```
 
+- `layout`: resolved layout used when the room was created. Late
+  joiners render the board from this. Optional for backward compat.
+
 When second player joins, server broadcasts `game.state` to BOTH players (initial board state).
 
 Error cases: