test: exercise engine snapshot updates under real parallelism

[abdallahsamabd]

Summary

Adds concurrency coverage for the policy engine so registration, hot reload, and the route cache are exercised the way a host actually uses them: one PolicyEngine behind Arc, invokes in flight, mutations on other threads.

The engine already serializes writers with runtime_write. This change does not touch that. It makes the test suite able to see real OS-thread overlap, assert that a successful register is not dropped, and check the generation / snapshot Release/Acquire pairing.

What changed

• Engine tests for registration, unregister, load_config / from_config, and the route cache now run with #[tokio::test(flavor = "multi_thread")].
• New seeded stress tests in crates/ppe-core/tests/engine_concurrency.rs:
  • eight threads barrier-register distinct plugins; every Ok must still be visible
  • N invoke tasks vs M mutators (register / unregister / annotate / reload); lost-update assertion, coherent invoke, cache refill after quiesce
  • replay with PPE_STRESS_SEED (optional PPE_STRESS_OPS, PPE_STRESS_INVOKERS, PPE_STRESS_MUTATORS)
• Loom model in crates/ppe-core/tests/loom_generation_snapshot.rs: if a reader Acquire-loads a higher generation, it must see the snapshot stored before the Release bump.
• Nightly ThreadSanitizer job (.github/workflows/nightly-tsan.yml, make test-tsan) runs the stress tests under TSan. PR CI is unchanged.
• CONTRIBUTING.md: when a new test needs flavor = "multi_thread".

Closes #22.

Test plan

• cargo test -p praxis-policy-core --test engine_concurrency --test loom_generation_snapshot
• cargo test -p praxis-policy-core --lib
• Optional: PPE_STRESS_SEED=12345 cargo test -p praxis-policy-core --test engine_concurrency stress_invoke -- --nocapture
• Confirm PR CI (make test) stays green; TSan is nightly-only