mark_sweep: add Gc::cast_ref_unchecked parity API and tests

[Flamki]

Part of #63

This PR adds Gc::cast_ref_unchecked parity support for both mark-sweep collectors and covers it with focused tests.

What changed

• Added unsafe fn Gc::cast_ref_unchecked<U>(&Gc<T>) -> &Gc<U> in:
  • mark_sweep
  • mark_sweep_arena2
• Added tests in both collectors:
  • cast_ref_unchecked_preserves_identity_and_value

Why

cast_ref_unchecked is still listed as a parity gap in #63.
This closes that gap without touching allocator/integration architecture.

Scope

• pointer API parity + tests only
• no allocator policy changes
• no collector lifecycle changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace -q
• cargo clippy --workspace --all-features --all-targets -q