refactor(V2): IBKR final review fixes — WS shutdown, conid match, clock note

Final code-review fixes:
- __main__: lifespan stops IBKRWebSocket singletons before registry close
- close_position: resolve symbol→conid first, match positions on conid
  (was matching contractDesc which is a long display string, not ticker)
- close_all_positions: prefer ticker field, fallback to contractDesc
- get_clock: explicit approximate=true + note about US holidays/half-days

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/__main__.py

@@ -9,6 +9,7 @@ Boot:
 """
 from __future__ import annotations
 
+import contextlib
 from contextlib import asynccontextmanager
 from typing import Literal, cast
 
@@ -54,6 +55,11 @@ def _make_app(settings: Settings) -> FastAPI:
         try:
             yield
         finally:
+            # Stop any IBKR WebSocket singletons before closing client registry
+            ibkr_ws_dict = getattr(app.state, "ibkr_ws", {}) or {}
+            for ws in ibkr_ws_dict.values():
+                with contextlib.suppress(Exception):
+                    await ws.stop()
             await app.state.registry.aclose()
 
     app.router.lifespan_context = lifespan