fix(sim-core): remove labor fill bias and market-order nondeterminism

[JoeHowarth]

Summary

• make labor assignment matching explicitly discrete and deterministic under partial fills by selecting unit winners using fill quantity and deterministic tie-breakers instead of zip pairing
• sort settlement merchant participation order before generating market orders so order IDs and tie-break outcomes are stable across runs with the same seed
• fix MerchantAgent::can_stockpile_at to reflect owned facility presence at a settlement and add unit test coverage

Validation

• cargo test -q