mark_sweep: run destructors for live values on collector drop

[Flamki]

Closes #20

This patch fixes collector teardown behavior so tracked live allocations are properly finalized and dropped when MarkSweepGarbageCollector itself is dropped.

What changed

• Added collector teardown cleanup to drain tracked queues and run finalization/drop hooks.
• Updated GC and ephemeron drop hooks to drop inner values (not just mark them dropped).
• Added regression test collector_drop_runs_destructors_for_live_gc_values.
• Kept the change focused to mark-sweep internals and tests.

Why

Arena::drop deallocates backing memory directly. Without dropping tracked live values first, destructors for GC-managed types may never run at collector teardown.

Validation

• cargo test -p oscars --lib -- --nocapture (33 passed)