rqe-iterator-suspend-resume

Model the iterator as one #[repr(C)] struct parametrized by an Active/Suspended ref_mode marker, reuse the heap allocation via Box::into_raw and Box::from_raw with a pointer cast, and derive resume decisions from the existing revalidate implementation. Add a standalone const layout-invariant proof and round-trip tests.

Use the review checklist: verify allocation reuse, child transitions routed through suspend_child_slot_in_place/resume_child_slot_in_place, every child slot transitioned including exhausted ones, teardown without dropping moved-from children, and behavioral parity between resume and the legacy revalidate.

Type-erased active and suspended children are different Box<dyn ...> types with different vtables, so a byte reinterpretation leaves the active vtable in a value typed as suspended, causing undefined behavior on the next dispatch. Children must be transitioned through the boxed.rs helper functions instead.

The FFI wrapper caches a raw pointer into the iterator and delegating wrappers hand out pointers into child storage, so the box address must survive the whole cycle. Rebuilding with Box::new would dangle those cached pointers.

No. Option<I>'s niche optimization makes its layout depend on I, so it is not transmute-stable across the active-to-suspended swap. Use a dedicated #[repr(C)] enum such as OptionalChild or MaybeEmptyOption instead.

Require a suspend-to-resume round-trip test including a type-erased child for wrappers, a box-address-stability assertion, and a steerable mock to exercise Moved, Aborted, and Err branches. The mock's suspended type must carry the real 'query lifetime rather than 'static.