initialize server project with basic structure and dependencies

Co-authored-by: Copilot <copilot@github.com>

README.md

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+# DiscoveryServer
+
+Lightweight Node.js + WebSocket server for ZAMNY co-op matchmaking.
+
+**Responsibilities**:
+
+- Player registry + nicknames (max 16 chars)
+- List available players
+- Room creation / joining (2-player co-op)
+- Relay for initial handshake
+
+**No game logic** (AD-03). All simulation runs client-side.
+
+## Quick Start
+
+```bash
+npm install
+npm run dev      # tsx watch, ws://localhost:3001
+npm test         # node:test via tsx
+npm run build
+npm start
+```
+
+## WebSocket Protocol
+
+All messages are JSON objects with `type`.
+
+### Client → Server
+
+- `{ "type": "set_nickname", "nickname": "Player1" }`
+- `{ "type": "list_players" }`
+- `{ "type": "join_room", "target": "p123" }` (TODO)
+
+### Server → Client
+
+- `{ "type": "welcome", "id": "p1" }`
+- `{ "type": "nickname_ok", "nickname": "Player1" }`
+- `{ "type": "player_list", "players": [{ "id": "p2", "nickname": "ZomSlayer" }] }`
+- `{ "type": "error", "message": "..." }`
+
+## Notes
+
+- Pure functions (`validateNickname`, `createRoomId`) are unit-tested.
+- Server only starts when run directly (not during tests).
+- Port 3001 (hardcoded for now).
+- See root [ARCHITECTURE.md](../ARCHITECTURE.md) for full design.