test: add property checks for runtime invariants

[AdmilsonCossa]

Summary

• add deterministic fast-check property tests for core WorkIt runtime invariants
• add npm run test:property
• include property tests in Vitest discovery so normal test and coverage gates exercise them

Invariants covered

• run.pool preserves order and never exceeds the concurrency cap
• work().inParallel preserves input order and respects the cap
• run.race cancels cooperative losers with typed race_lost reasons
• run.retry stops immediately on CancellationError
• run.timeout preserves typed timeout reasons
• first scope cancellation reason remains authoritative

Validation

• npm run test:property
• npm test
• npm run test:coverage
• npm run verify

Closes #19