fix: add threading lock to graph cache, expand docstring

Address review feedback from @bensig:
1. Wrap cache reads/writes in threading.Lock for thread safety
2. Promote the col-arg caveat from inline comment to docstring

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jphein

mempalace/palace_graph.py

@@ -15,6 +15,7 @@
 No external graph DB needed — built from ChromaDB metadata.
 """
 
+import threading
 import time
 from collections import defaultdict, Counter
 
@@ -23,6 +24,7 @@
 
 # Module-level graph cache with TTL and write-invalidation.
 # Warm cache serves build_graph() in O(1); invalidate_graph_cache() clears on writes.
+_graph_cache_lock = threading.Lock()
 _graph_cache_nodes = None
 _graph_cache_edges = None
 _graph_cache_time = 0.0
@@ -32,9 +34,10 @@
 def invalidate_graph_cache():
     """Clear the graph cache. Called from mcp_server.py on writes."""
     global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
-    _graph_cache_nodes = None
-    _graph_cache_edges = None
-    _graph_cache_time = 0.0
+    with _graph_cache_lock:
+        _graph_cache_nodes = None
+        _graph_cache_edges = None
+        _graph_cache_time = 0.0
 
 
 def _get_collection(config=None):
@@ -54,7 +57,11 @@ def build_graph(col=None, config=None):
     Build the palace graph from ChromaDB metadata.
 
     Returns cached result if fresh (within TTL). Cache is invalidated
-    on writes via invalidate_graph_cache().
+    on writes via invalidate_graph_cache(). Thread-safe via _graph_cache_lock.
+
+    Note: warm cache ignores ``col`` and ``config`` arguments — this is
+    intentional for the MCP server's single-palace use case. Callers
+    switching collections should call ``invalidate_graph_cache()`` first.
 
     Returns:
         nodes: dict of {room: {wings: set, halls: set, count: int}}
@@ -64,8 +71,9 @@ def build_graph(col=None, config=None):
     now = time.time()
     # NOTE: warm cache ignores col/config args — intentional for the MCP server's
     # single-palace use case. Callers switching collections must invalidate first.
-    if _graph_cache_nodes is not None and (now - _graph_cache_time) < _GRAPH_CACHE_TTL:
-        return _graph_cache_nodes, _graph_cache_edges
+    with _graph_cache_lock:
+        if _graph_cache_nodes is not None and (now - _graph_cache_time) < _GRAPH_CACHE_TTL:
+            return _graph_cache_nodes, _graph_cache_edges
 
     if col is None:
         col = _get_collection(config)
@@ -125,9 +133,10 @@ def build_graph(col=None, config=None):
     # Only cache non-empty graphs so new data is picked up immediately
     # when the palace is first populated.
     if nodes:
-        _graph_cache_nodes = nodes
-        _graph_cache_edges = edges
-        _graph_cache_time = time.time()
+        with _graph_cache_lock:
+            _graph_cache_nodes = nodes
+            _graph_cache_edges = edges
+            _graph_cache_time = time.time()
 
     return nodes, edges