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.
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