test: expand coverage to 70%, fix mcp_server CI crash (threshold 60%)

Add/expand tests for normalize (39%→97%), searcher (39%→100%),
layers (28%→97%), split_mega_files (34%→72%).

Fix mcp_server.py parse_args→parse_known_args to prevent SystemExit
when imported during pytest (CI was crashing on all test jobs).

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

mempalace/mcp_server.py

@@ -44,7 +44,8 @@ def _parse_args():
         metavar="PATH",
         help="Path to the palace directory (overrides config file and env var)",
     )
-    return parser.parse_args()
+    args, _ = parser.parse_known_args()
+    return args
 
 
 _args = _parse_args()