test(symbol-library): init _cache_lock in __new__-based fixture

The background warm-cache change made list_symbols() guard cache writes with
self._cache_lock. _manager_for_fixture() builds the manager via __new__ (to skip
disk I/O to system libs), bypassing __init__ where the lock is created, so the 8
SPICE-parsing tests raised AttributeError. Set the lock in the fixture. The
string-aware parser itself is correct — all 8 pass once the lock exists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_symbol_library.py

@@ -7,6 +7,7 @@ Covers:
 """
 
 import sys
+import threading
 from pathlib import Path
 
 import pytest
@@ -24,6 +25,9 @@ def _manager_for_fixture() -> SymbolLibraryManager:
     manager.project_path = None
     manager.libraries = {"Simulation_SPICE": str(FIXTURE)}
     manager.symbol_cache = {}
+    # list_symbols() guards cache writes with this lock (added alongside the
+    # background warm-cache). The fixture bypasses __init__, so set it here.
+    manager._cache_lock = threading.Lock()
     return manager