PINK: cancel_async, S2 task guard, 29 new regression tests — 346/346 green

Bug fixes:
  1. bingx_venue.py: add cancel_async() — async cancel that awaits backend.cancel()
     directly in the main event loop. The sync cancel() path goes through _run()
     → thread-pool → asyncio.run() in a new thread, but aiohttp is bound to the
     main loop → deadlock. Identical root cause as the old sync submit() → fixed
     via submit_async. Remove dead cancel_order branch (BingxDirectExecutionAdapter
     has cancel, not cancel_order).

  2. rust_backend.py: process_intent_async CANCEL path now uses cancel_async when
     available (matching the submit_async pattern for ENTER/EXIT). Sync cancel()
     fallback kept for MockVenueAdapter compat.

  3. bingx_direct.py: guard S2 background refresh task per symbol. Old code discarded
     the task reference; rapid submits piled up concurrent _refresh_state_background
     calls all writing self._state in arbitrary completion order (stale last-writer-
     wins). Now: skip creating a new task if one is already pending for the symbol;
     store reference and clear via done-callback.

Test additions (test_bingx_bugs.py, 29 tests):
  - cancel_async: awaitable, calls backend.cancel directly, maps all statuses
  - process_intent_async CANCEL: dispatches cancel_async / falls back to sync
  - S2 guard: task stored, no duplicates while pending, new task after done
  - _events_from_submit with None snapshots: FILLED/NEW/REJECTED/PARTIAL/RATE_LIMITED
  - _filled_size_from_snapshots(None, None): safe 0.0 return
  - _events_from_cancel: before/after completely ignored
  - connect(): no double refresh_state, no-op if backend has no connect
  - submit() sync with None snapshots: FULL_FILL still emitted
  - cancel() branch audit: uses cancel not cancel_order, raises for no-cancel backend

Fix: test_exchange_event_seam_parity.py TestMockSubscribe — replace deprecated
asyncio.get_event_loop().run_until_complete() with asyncio.run() (Python 3.12
raises RuntimeError when event loop is closed by earlier suite tests).

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

prod/clean_arch/dita_v2/bingx_venue.py

@@ -348,12 +348,29 @@ class BingxVenueAdapter(VenueAdapter):
         # backend.connect() already called refresh_state() — no second fetch needed
         return True
 
+    async def cancel_async(self, order: VenueOrder, *, reason: str = "") -> List[VenueEvent]:
+        """Async cancel — runs in the caller's event loop, no thread-pool deadlock.
+
+        The sync cancel() path goes through _call_backend → _run → thread-pool →
+        asyncio.run() in a new thread.  The aiohttp session is bound to the main
+        event loop, so using it from a different loop deadlocks — same bug that
+        was fixed for submit via submit_async.  This version awaits backend.cancel()
+        directly in the caller's (main) event loop.
+        """
+        cancel_fn = getattr(self.backend, "cancel", None)
+        if cancel_fn is not None:
+            response = await cancel_fn(order, reason=reason)
+        else:
+            response = None
+        return self._events_from_cancel(order, response, None, None, reason=reason)
+
     def cancel(self, order: VenueOrder, *, reason: str = "") -> List[VenueEvent]:
-        # _events_from_cancel never reads before/after — snapshots are dead weight
+        # _events_from_cancel never reads before/after — snapshots are dead weight.
+        # NOTE: if backend.cancel is async (BingxDirectExecutionAdapter), this sync
+        # path goes through the thread-pool and will deadlock in a running event loop.
+        # Use cancel_async() from async contexts (process_intent_async already does).
         response = None
-        if hasattr(self.backend, "cancel_order"):
-            response = self._call_backend("cancel_order", order, reason=reason)
-        elif hasattr(self.backend, "cancel"):
+        if hasattr(self.backend, "cancel"):
             response = self._call_backend("cancel", order, reason=reason)
         else:
             client = getattr(self.backend, "_client", None)