Chat sample: User-created rooms at runtime

[pathosDev]

Followup from the chat sample (`examples/chat/`). The current sample
hardcodes the room list in `shared/rooms.ts`; this issue tracks
making rooms dynamic.

Approach

Replace `DEFAULT_ROOMS` with a DistributedData-backed registry:

```ts
const ROOMS_KEY = 'chat.rooms';
dd.update<ORSet>(ROOMS_KEY, () => ORSet.empty(), s => s.add(replicaId, name));
```

Protocol additions

```ts
type ClientMessage = ... | { type: 'create-room', name: string };
type ServerMessage = ... | { type: 'room-added', name: string }
| { type: 'room-removed', name: string };
```

UI

• "+ New room" button in the rooms panel.
• Subscribe to ORSet changes → push `room-added`/`room-removed` to all clients.

Out of scope (followup)

• Per-room access control (private rooms).
• Room deletion + history archival.

Source: `examples/chat/` README → "Out of scope" section.

[pathosDev]

Implemented in 33c7315 (feat(chat): user-created rooms at runtime via DistributedData ORSet). Closing once the branch is pushed; the commit's Closes #98 trailer would have auto-closed otherwise.