[sparkysolutions10-pixel]

Closes #335

Summary of Changes

• Enforces WebSocket backpressure and memory bounds in src/ws/fanout.ts based on socket bufferedAmount.
• Configurable two-tier threshold:
  1. Soft Threshold (WS_BUFFER_THRESHOLD_BYTES, default 64KB): When a subscriber's buffer exceeds this threshold, intermediate price updates are coalesced and dropped to prevent memory queue growth, incrementing ws_messages_dropped_total.
  2. Hard Ceiling (WS_BUFFER_MAX_BYTES, default 1MB): Stalled peers exceeding this ceiling are terminated with WebSocket close code 1008 ("Buffer overflow") and unregistered, incrementing ws_slow_clients_total.
• Guarantees that slow or stalled subscribers never exhaust server heap or degrade delivery to healthy peers.
• Only increments ws_messages_sent_total upon confirmed write dispatch.
• Added automated unit tests in src/__tests__/fanout.test.ts.