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/adapters/bingx_direct.py

@@ -226,6 +226,11 @@ class BingxDirectExecutionAdapter(ExecutionPort):
 
         # ── S2: Background state refresh tracking ─────────────────────────────
         self._state_refreshed_at: float = 0.0   # monotonic seconds
+        # Per-symbol pending background-refresh task.  Prevents concurrent REST
+        # calls piling up (rapid submits) — if a task is already running for a
+        # symbol we skip creating a new one; the running task captures state after
+        # the most recent fill anyway.  Done callback removes the entry on completion.
+        self._s2_tasks: Dict[str, "asyncio.Task[None]"] = {}
 
     @property
     def state(self) -> ExchangeStateSnapshot | None:
@@ -706,10 +711,14 @@ class BingxDirectExecutionAdapter(ExecutionPort):
         # For LIMIT / non-FILLED orders: must refresh synchronously to detect resting order.
         market_filled = (status == "FILLED" and not is_limit)
         if market_filled:
-            asyncio.create_task(
-                self._refresh_state_background(intent.asset),
-                name=f"state_refresh_{symbol}",
-            )
+            existing = self._s2_tasks.get(symbol)
+            if existing is None or existing.done():
+                task = asyncio.create_task(
+                    self._refresh_state_background(intent.asset),
+                    name=f"state_refresh_{symbol}",
+                )
+                self._s2_tasks[symbol] = task
+                task.add_done_callback(lambda _t, _s=symbol: self._s2_tasks.pop(_s, None))
         else:
             self._state = await self._refresh_exchange_state(intent.asset, include_history=False)