Cache type index with mtime-based invalidation

_load_type_index() now caches the parsed dict at module level and only
re-reads type-index.txt when its mtime changes. This avoids a file read
and parse on every get_class_overview / get_member call. The cache is
automatically invalidated if the docs are regenerated.

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

ue_mcp_server.py

@@ -28,15 +28,24 @@ def _engine_root() -> Path:
     return Path(p)
 
 
+_type_index_cache: dict[str, str] = {}
+_type_index_mtime: float = 0.0
+
+
 def _load_type_index() -> dict[str, str]:
-    """Returns {TypeName: relative_md_path}."""
-    idx = (_docs_root() / "type-index.txt").read_text()
+    """Returns {TypeName: relative_md_path}, reloading only if the file has changed."""
+    global _type_index_cache, _type_index_mtime
+    idx_path = _docs_root() / "type-index.txt"
+    mtime = idx_path.stat().st_mtime
+    if mtime != _type_index_mtime:
         result = {}
-    for line in idx.splitlines():
+        for line in idx_path.read_text().splitlines():
             if ": " in line:
                 name, path = line.split(": ", 1)
                 result[name.strip()] = path.strip()
-    return result
+        _type_index_cache = result
+        _type_index_mtime = mtime
+    return _type_index_cache
 
 
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