Make L2 exchange response ordering deterministic

[California-a]

Summary

• make the L2 PartialFillExchange and NoPartialFillExchange order containers deterministic
• preserve an explicit ascending OrderId response order when multiple fills share a timestamp
• add regression tests that exercise the real exchange response bus for both models

Why

The default Rust HashMap and HashSet use randomized seeds and expose arbitrary iteration order. Both L2 exchange models emitted responses while iterating those containers. When several fills had the same timestamp, identical inputs could therefore append responses in different orders across processes.

OrderBus intentionally preserves append order for equal timestamps, so the randomized traversal became externally observable and could change downstream order-lifecycle handling.

Using BTreeMap and BTreeSet gives the response path a stable ascending OrderId order without changing the public API.

Validation

• cargo test -p hftbacktest --lib --no-default-features --features backtest --offline: 24 passed
• each new regression test repeats the simultaneous three-order response case 32 times
• patch-scoped Clippy passed with only the repository's existing unrelated lint categories allowed
• git diff --check passed

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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