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/chess/src/index.ts

@@ -38,3 +38,25 @@ export {
   type PresetActivation,
   type PresetScope,
 } from "./presets/active-set.js";
+
+// Starting Layouts — piece-placement abstractions orthogonal to
+// preset rules. Consumed by the server for room.create layout
+// resolution and by the client Lobby for premade selection.
+export {
+  LAYOUT_REGISTRY,
+  CLASSIC_LAYOUT,
+  EMPTY_LAYOUT,
+  DUNSANY_LAYOUT,
+  MONSTER_LAYOUT,
+  PAWNS_ONLY_LAYOUT,
+  HORDE_LAYOUT,
+  KNIGHTMATE_LAYOUT,
+  CHESS960_SHIM,
+  buildChess960Layout,
+  toFen,
+  fromFen,
+  type PiecePlacement,
+  type StartingLayout,
+  type LayoutValidationResult,
+} from "./layouts/index.js";
+export { validateLayout } from "./layouts/validate.js";