gc: add finalizer_safe() parity API across mark-sweep collectors

[Flamki]

Part of #78 and #63.

This PR adds finalizer_safe() runtime utility parity in both mark-sweep collectors and validates its behavior with focused tests.

What changed

• Added MarkSweepGarbageCollector::finalizer_safe(&self) -> bool in:
  • collectors/mark_sweep/mod.rs
  • collectors/mark_sweep_arena2/mod.rs
• Added parity tests in both collector test suites:
  • finalizer_safe_reflects_collecting_state

Behavior validated by tests

• finalizer_safe() returns true while idle.
• finalizer_safe() returns false when observed from inside Trace::trace during an active collection cycle.
• After collection completes, finalizer_safe() returns true again.

Scope

• runtime utility parity + tests only
• no allocator policy changes
• no collector algorithm changes
• no Boa integration wiring changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace -q
• cargo clippy --workspace --all-features --all-targets -q
• cargo +nightly miri test -p oscars --all-features -q