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>

pyproject.toml

@@ -69,7 +69,7 @@ testpaths = ["tests"]
 source = ["mempalace"]
 
 [tool.coverage.report]
-fail_under = 50
+fail_under = 60
 show_missing = true
 exclude_lines = [
     "if __name__",