feat(gems): add multiplayer gem game with custom content packs

- implement FastAPI and HTMX lobby and game interfaces
- support up to four human and AI-controlled players
- add configurable rules, victory conditions, and expansion modules
- support validated JSON cards, patrons, objectives, and outposts

python/gems/events.py

@@ -0,0 +1,35 @@
+"""In-process revision notifications for SSE clients."""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+from collections import defaultdict
+
+
+class EventBroker:
+    """Fan out room revisions; clients always reload the latest snapshot."""
+
+    def __init__(self) -> None:
+        """Initialize an empty subscription registry."""
+        self._queues: dict[str, set[asyncio.Queue[int]]] = defaultdict(set)
+
+    def subscribe(self, room_code: str) -> asyncio.Queue[int]:
+        """Subscribe a bounded notification queue to a room."""
+        queue: asyncio.Queue[int] = asyncio.Queue(maxsize=1)
+        self._queues[room_code].add(queue)
+        return queue
+
+    def unsubscribe(self, room_code: str, queue: asyncio.Queue[int]) -> None:
+        """Remove a room notification queue from the registry."""
+        self._queues[room_code].discard(queue)
+        if not self._queues[room_code]:
+            self._queues.pop(room_code, None)
+
+    def publish(self, room_code: str, revision: int) -> None:
+        """Publish the newest room revision to every subscriber."""
+        for queue in tuple(self._queues.get(room_code, ())):
+            if queue.full():
+                with contextlib.suppress(asyncio.QueueEmpty):
+                    queue.get_nowait()
+            queue.put_nowait(revision)