Feat/gc drop finalizer parity

[Flamki]

This PR tightens mark-sweep teardown/finalization semantics with minimal scope.

What changed

• Added a typed-erased finalizer hook to the GC vtable (FinalizeFn) so GcBox<T> finalizers can be dispatched correctly from erased pointers.
• Updated collector teardown (sweep_all_queues) to run finalize_fn before drop_fn for:
  • root_queue
  • pending_root_queue
  • ephemeron_queue
  • pending_ephemeron_queue
• Updated dead-root sweep path to use the new vtable finalizer dispatch.
• Updated ephemeron finalizer hook to use Trace::run_finalizer(...).
• Added regression tests:
  • collector_drop_runs_finalizers_for_live_gc_values
  • collector_drop_runs_ephemeron_value_finalizers_for_live_values

Why

Previous teardown behavior reliably dropped live tracked allocations, but finalizer execution for erased GC values was incomplete/inconsistent. This patch keeps the PR focused to mark-sweep internals and guarantees finalizers are run during collector teardown where safe.

Scope

• mark-sweep internals + tests only
• no public API changes
• no allocator policy/threshold changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-features --all-targets
• cargo clippy --workspace --no-default-features --all-targets
• cargo doc -v --document-private-items --all-features