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)

[Flamki]

Update from my side (no code review from maintainers yet, just technical status):

I rebased this branch onto latest main and resolved the mark-sweep merge conflicts.

I also kept the PR intentionally narrow. Relative to current main, this branch now changes only:

• oscars/src/collectors/mark_sweep/mod.rs
• oscars/src/collectors/mark_sweep/tests.rs

Technical behavior change:

• In MarkSweepGarbageCollector::drop, I now check whether any queued GC nodes are still rooted (is_rooted() on root_queue and pending_root_queue).
• If rooted values still exist, we keep the existing leak-protection path (avoid potential UAF).
• If no rooted values remain, we run sweep_all_queues() so tracked GC/ephemeron allocations are dropped during collector teardown.

Regression coverage added:

• collector_drop_runs_destructors_for_live_gc_values
• collector_drop_runs_ephemeron_value_destructors_for_live_values

Validation run locally:

• cargo fmt --all -- --check
• cargo test --workspace (all passing: 48 tests in oscars + doc tests)

If you want, I can also split this into a follow-up cleanup PR, but functionally this branch is now focused on collector-drop teardown semantics.