perf: graph cache with write-invalidation in build_graph()

build_graph() scans every drawer's metadata in 1000-item batches on
every call — O(n) per graph build with no caching. At 50K+ drawers
this costs several seconds per MCP tool call (traverse, find_tunnels,
graph_stats all call build_graph on every invocation).

Add a module-level cache (nodes + edges + timestamp) with a 60-second
TTL. Cache is invalidated via invalidate_graph_cache(), exported for
write operations to call. Tests updated with setup_method cache resets
and two new tests verifying cache hit and invalidation behaviour.

mempalace/palace_graph.py

@@ -18,6 +18,7 @@ No external graph DB needed — built from ChromaDB metadata.
 import hashlib
 import json
 import os
+import time
 from collections import Counter, defaultdict
 from datetime import datetime, timezone
 
@@ -25,6 +26,20 @@ from .config import MempalaceConfig
 from .palace import get_collection as _get_palace_collection
 from .palace import mine_lock
 
+# Module-level graph cache — mirrors _metadata_cache pattern in mcp_server.py
+_graph_cache_nodes = None
+_graph_cache_edges = None
+_graph_cache_time = 0.0
+_GRAPH_CACHE_TTL = 60.0  # seconds — graph changes less often than metadata
+
+
+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
+
 
 def _get_collection(config=None):
     config = config or MempalaceConfig()
@@ -42,10 +57,18 @@ 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().
+
     Returns:
         nodes: dict of {room: {wings: set, halls: set, count: int}}
         edges: list of {room, wing_a, wing_b, hall} — one per tunnel crossing
     """
+    global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
+    now = time.time()
+    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)
     if not col:
@@ -101,6 +124,10 @@ def build_graph(col=None, config=None):
             "dates": sorted(data["dates"])[-5:] if data["dates"] else [],
         }
 
+    _graph_cache_nodes = nodes
+    _graph_cache_edges = edges
+    _graph_cache_time = time.time()
+
     return nodes, edges