Fence an agent away from fragile C++ code

An agent is about to add a new packet-parsing path to your C++ networking code. 2 rules matter, and neither is visible in the code. The hot path must never allocate on the heap. The legacy/ directory is fragile and must not be touched.

The last time an agent worked here, it used a std::vector as a parse buffer, and the latency regression took a week to trace.

How do you set up the work so the agent stays out of legacy/ and keeps the hot path free of allocations?