tests(mark_sweep): add coverage for merged parity helpers

[Flamki]

This PR adds focused test coverage for the parity helper APIs that were merged from #64, #65, and #66.

Refs: #63

What this adds

• mark_sweep tests for:
  • Gc::into_raw / Gc::from_raw round-trip identity and value stability
  • Gc::downcast / Gc::cast_unchecked parity behavior
  • WeakGc::upgrade liveness behavior before and after collection
• Matching coverage in mark_sweep2 (mark_sweep_arena2) for the same helper set

Why

In #63, maintainer feedback called out that parity was in a good place but these newly added helpers still needed tests.

This PR keeps scope strictly to tests for those merged parity APIs.

Scope

• tests only
• no runtime or API behavior changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace