test(orders): cover multi-tier exit precedence

[luisleo526]

Summary

• add focused unit coverage for multi-tier strategy.exit precedence
• covers sibling limit tiers, shared stops, mixed limit-then-stop paths, gap-through open fills, and partial-position remainder behavior
• documents that low probe price-exact rate was verifier pairing artifact from TV margin-call trades, not an engine fill-precedence bug

Verification

• ctest --output-on-failure: 74/74 passed
• scripts/check_c_abi_runtime.py
• scripts/run_corpus.sh: 252 strategies, excellent=251, anomaly=1, fail=0