gc: add finalizer_safe() runtime utility parity across collectors

[Flamki]

Context

• #63 still lists finalizer_safe() as an open runtime utility parity gap.
• Current collectors already track collection state (is_collecting), but there is no unified public parity surface for this utility.

Problem

• Boa-facing API compatibility expects a finalizer_safe() style check.
• Without this, integration work in #26/#28 lacks a direct parity hook for finalizer-phase safety checks.

Proposed scope

1. Define and document finalizer_safe() parity behavior for Oscars collectors.
2. Add the API in both mark_sweep and mark_sweep_arena2 surfaces with consistent semantics.
3. Add focused tests proving expected behavior during and outside collection/finalization-sensitive paths.
4. Update parity tracking in #63 once implemented.

Non-goals

• No collector algorithm changes.
• No allocator policy changes.
• No broad Boa integration wiring in this issue.

Acceptance criteria

• finalizer_safe() is available with consistent behavior in both collectors.
• Tests cover true/false behavior around collection state transitions.
• Workspace checks pass (fmt, test, clippy, and miri where applicable).