Integer attestations keep their declared class; model-slot writes enter the boundary check

- integer_slots records u64 as u64: the wire flattening narrows to the frozen TypeRef spellings where it belongs
- Declaring any integer slot force-lowers the designated init, update, and subscription exports, so every model-field write proves or refuses like an export slot; declared field reads seed their class shapes
- Export integer classes stay attested by the emitted C signature under the same gate — the sidecar list is the document-side complement, and its refusal message now says so