[Exchange-3.2] Concurrent order ingestion

[vaasu2002]

As the exchange, I want to ingest orders from many clients concurrently without blocking,
so that high throughput and low latency are maintained.

Acceptance:

• Acceptor thread distributes sockets to worker threads.
• Worker threads handle multiple client sockets concurrently.
• No shared locks on the hot path other than per-thread state.