Add fuzz and property-based tests for core invariants

[AdmilsonCossa]

Goal

Add fuzz and property-based testing for WorkIt's core runtime invariants after the current 0.1.2 release hardening work is complete.

Candidate invariants

• cancellation is idempotent
• cancelled scopes do not admit new owned children
• sibling failure cancels owned siblings under fail-fast policy
• retries stop after cancellation
• timeout reasons remain typed and observable
• run.race losers receive race_lost
• pool concurrency never exceeds the configured cap
• stream backpressure does not over-pull beyond the asserted bound

Discipline

• use public runtime paths, not private helper-only paths
• keep generators bounded and self-terminating
• run in CI only if deterministic enough for normal pull requests
• map any promoted proof to evidence/claims.json