feat: add WebSocket order book feed with reconnection and heartbeat

[alexanderxfgl-bit]

Closes #7

What

Adds a transport-agnostic WebSocket order book feed system that allows strategies to consume real-time L2 data without caring about the underlying transport.

Changes

• src/connectors/orderbook_feed.py — Complete order book feed system:

  • BaseOrderBookFeed — Abstract interface with subscribe(), unsubscribe(), get_orderbook(), and on_update() callbacks. Strategies depend on this, not specific transports.

  • WebSocketOrderBookFeed — Core WebSocket transport:

    • Exponential backoff reconnection (configurable base/max delay, max attempts)
    • Heartbeat monitoring via ping_interval and ping_timeout
    • Snapshot + incremental delta merging (snapshot replaces, delta updates)
    • Delta queuing: deltas received before initial snapshot are buffered
    • Zero-quantity level removal (handles filled/cancelled orders)
    • Auto re-subscribe on reconnect
    • wait_for_snapshot() async helper for initial data

  • BinanceOrderBookFeed — Binance partial book depth stream:

    • Supports depth 5/10/20 at 100ms or 1000ms intervals
    • SUBSCRIBE/UNSUBSCRIBE protocol messages
    • Parses Binance depthUpdate format

  • UniswapV3OrderBookFeed — Reconstructed order book from on-chain data:

    • Takes slot0 (sqrtPriceX96, liquidity, tick) as input
    • Reconstructs L2 at adjacent ticks with decayed liquidity
    • Useful for strategies that need simulated order book from Uniswap

  • OrderBookSnapshot — Rich data model:

    • best_bid, best_ask, spread, mid_price properties
    • depth_at_price() for analyzing liquidity at a level
    • Sequence tracking for delta ordering

  • create_binance_feed() — Factory function for quick setup

Tests

28 new tests covering:

• OrderBookSnapshot properties (empty, best_bid/ask, spread, mid_price, depth)
• WebSocket feed subscribe/unsubscribe and symbol normalization
• Full snapshot replacement
• Delta queuing before snapshot + application after
• Zero-quantity level removal
• Static level update helper
• Binance message parsing + non-depth event filtering
• Uniswap V3 slot0 reconstruction
• Factory function and config defaults

All 106 tests passing (1 pre-existing integration failure).