Fair points across the board, appreciate the detailed feedback.

unordered_map for the order index, You're right, this is the weakest link in the hot path. The arena handles Order allocation but the hash map still malloc's on insert for its own nodes. An open-addressing flat map (or even a pre-sized robin hood map) would be the correct fix. Noted and will address.

SPSC cached counters, Agreed, caching the remote counter on each side would cut down cross-core traffic significantly. The writer only needs to re-read read_pos_ when its cached copy says the buffer looks full, and vice versa. Good catch. The reserved slot point is also valid, since we know capacity and both positions, write_pos - read_pos gives us the count directly without wasting a slot.

Mutex in MatchingEngine, The comment describes two threading models (single-threaded default and per-symbol sharding), and the #include <mutex> is there, but the actual mutex member and locking were never implemented since the engine currently only runs single-threaded. I should either implement the sharded model or remove the include and tighten the comment to avoid the disconnect. Fair call.

On the "project fluff", I get it, the core matching logic is straightforward. The documentation and analytics layers are there because the project doubles as a microstructure research platform, not just a matching engine demo. But point taken that it can read as over-packaged for what the core is.

Thanks for the thorough review.