mark_sweep: add Gc::new_cyclic_in parity semantics and tests

[Flamki]

Part of #63

This PR adds Gc::new_cyclic_in parity support for both mark-sweep collectors, following Boa semantics where weak.upgrade() returns None inside the construction closure until the object is fully initialized.

What changed

• Added Gc::new_cyclic_in in:
  • mark_sweep::pointers::Gc
  • mark_sweep_arena2::pointers::Gc
• Added empty ephemeron allocation path used by cyclic bootstrap:
  • alloc_empty_ephemeron_node in mark-sweep collector APIs
  • matching arena2 collector allocation helper
• Added internal helpers to support deferred key binding:
  • WeakGcBox::new_empty
  • Ephemeron::<K, ()>::new_empty
  • Ephemeron::<K, ()>::set_key
  • WeakGc::from_raw + WeakGc::set_key (crate-visible)
• Added parity tests in both collectors:
  • new_cyclic_closure_cannot_upgrade_before_init

Why

Gc::new_cyclic is still listed as a parity gap in #63.
This closes that gap while keeping scope limited to mark-sweep internals and tests.

Scope

• API parity + tests only
• no allocator policy tuning
• no integration path changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace -q
• cargo clippy --workspace --all-features --all-targets -q

[Flamki]

Rebased on latest main and resolved conflicts.
All CI checks are green again.
Still part of #63; ready for review when you get a chance.