PINK Phase 2 (G3): ExchangeEvent seam + BingxUserStream + mode-parity

exchange_event.py: abstract ExchangeEvent/ExchangeEventKind seam
venue.py: VenueAdapter extended with subscribe()/account_snapshot()
bingx_user_stream.py: PINK-only WS client with listenKey lifecycle,
  gzip, ping/pong, 24h rotation sentinel, reconnect backoff, gap-backfill
mock_venue.py: subscribe()/account_snapshot() for offline tests

Gate G3 mode-parity: WS and poll paths produce identical k_capital,
fees, realized PnL, reconcile status for same logical event sequence.
89/89 total offline tests pass.

prod/clean_arch/dita_v2/venue.py

@@ -0,0 +1,60 @@
+"""Venue adapter contracts for DITAv2."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import Any, AsyncIterator, Dict, List, Optional, Protocol
+
+from .contracts import (
+    KernelCommandType,
+    KernelIntent,
+    KernelEventKind,
+    TradeSide,
+    VenueEvent,
+    VenueEventStatus,
+    VenueOrder,
+    VenueOrderStatus,
+)
+from .exchange_event import ExchangeEvent
+
+
+class VenueAdapter(Protocol):
+    """Abstract venue adapter used by the kernel."""
+
+    def submit(self, intent: KernelIntent) -> List[VenueEvent]:
+        ...
+
+    def cancel(self, order: VenueOrder, *, reason: str = "") -> List[VenueEvent]:
+        ...
+
+    def open_orders(self) -> List[VenueOrder]:
+        ...
+
+    def open_positions(self) -> List[Dict[str, Any]]:
+        ...
+
+    def reconcile(self) -> List[VenueEvent]:
+        ...
+
+    # ------------------------------------------------------------------
+    # Phase 2 — stream seam (spec G3)
+    # ------------------------------------------------------------------
+
+    async def subscribe(self) -> AsyncIterator[ExchangeEvent]:
+        """
+        Yield ExchangeEvent instances in arrival order.  Implementations
+        must handle reconnection, keepalive, and 24h rotation internally.
+        The iterator never terminates normally — callers cancel it on
+        shutdown.  Both the WS and poll-failover paths implement this
+        interface so the kernel layer is source-agnostic.
+        """
+        ...  # pragma: no cover
+
+    async def account_snapshot(self) -> ExchangeEvent:
+        """
+        Return a single ACCOUNT_UPDATE + POSITION_UPDATE merged event
+        by calling the exchange REST API.  Used for gap-backfill on
+        reconnect and as the poll-failover path.
+        """
+        ...  # pragma: no cover