Add hot-path allocation counter feature for benchmarking

[joaquinbejar]

Overview

Add a feature-gated allocation counter on the hot path (ingest → execution report) to measure and report allocation count. Use a wrapper allocator or counters to identify unnecessary allocations. Gate behind feature = "alloc-counters" to avoid production overhead.

Context

• Files: Cargo.toml, src/lib.rs, src/orderbook/operations.rs
• Worktest requirement: "Allocation count on the hot path (should be zero or near-zero)"
• Signal: staff-level engineering; zero-alloc paths are rare but valuable
• Optional: CPU pinning to reduce variance

Acceptance criteria

• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo fmt --all --check clean
• cargo test --all-features clean
• cargo build --release clean
• cargo test --all-features --test '*alloc*' runs without error
• Feature flag compiles cleanly without affecting production binary size
• Unit test: test_ingest_zero_alloc confirms allocation count ≤ expected threshold
• BENCH.md notes allocation count alongside latency p99.99

[joaquinbejar]

Duplicate of #58, tracked there.

[joaquinbejar]

Duplicate of #58 — already implemented by PR #79 (alloc counter). Closing as not planned.