PINK: FSM rollback on venue.submit failure via synthetic REJECTED event

When venue.submit() raises (BingX timeout / network error), the Rust FSM
had already advanced to ORDER_REQUESTED/ENTRY_WORKING with no corresponding
exchange order — stranding the slot. Every subsequent ENTER for a different
asset hit SLOT_BUSY, preventing recovery without a restart. Restarts create
a fresh IDLE kernel, leaving the orphaned exchange position unmanaged.

Fix: catch submit exceptions, synthesise an ORDER_REJECT VenueEvent, feed it
through on_venue_event() so the FSM rolls back to IDLE atomically. The slot
is free on the next cycle with no orphan on the exchange.

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

prod/clean_arch/dita_v2/rust_backend.py

@@ -793,7 +793,39 @@ class ExecutionKernel:
         emitted_events = []
         all_venue_transitions: List[KernelTransition] = []
         if outcome.accepted and intent.action in {KernelCommandType.ENTER, KernelCommandType.EXIT}:
-            emitted_events = self.venue.submit(intent)
+            try:
+                emitted_events = self.venue.submit(intent)
+            except Exception as _submit_exc:
+                # venue.submit() failed (e.g. BingX timeout). The Rust FSM already
+                # advanced to ORDER_REQUESTED / ENTRY_WORKING with no corresponding
+                # exchange order. Feed a synthetic REJECTED event so the FSM rolls
+                # back to IDLE — otherwise the slot is stranded and every subsequent
+                # ENTER with a different trade_id hits SLOT_BUSY forever.
+                import logging as _log
+                _log.getLogger(__name__).error(
+                    "venue.submit failed (%s) — feeding synthetic REJECTED to roll back FSM slot=%d action=%s",
+                    _submit_exc, intent.slot_id, intent.action.value,
+                )
+                _reject_event = VenueEvent(
+                    timestamp=datetime.now(timezone.utc),
+                    event_id=f"{intent.trade_id}:submit_error",
+                    trade_id=intent.trade_id,
+                    slot_id=intent.slot_id,
+                    kind=KernelEventKind.ORDER_REJECT,
+                    status=VenueEventStatus.REJECTED,
+                    venue_order_id="",
+                    venue_client_id="",
+                    side=intent.side,
+                    asset=intent.asset,
+                    price=0.0,
+                    size=float(intent.target_size or 0.0),
+                    filled_size=0.0,
+                    remaining_size=float(intent.target_size or 0.0),
+                    reason=f"VENUE_SUBMIT_ERROR:{_submit_exc}",
+                    raw_payload={},
+                    metadata={"intent_id": intent.intent_id, "action": intent.action.value},
+                )
+                emitted_events = [_reject_event]
             for event in emitted_events:
                 evt_outcome = self.on_venue_event(event)
                 all_venue_transitions.extend(evt_outcome.transitions)