Record declared u64 classes in the sidecar attestation and prove model/msg slot writes

- integer_slots now records each slot's DECLARED class ({i64, u64}); the i64 flattening narrows to the frozen format-1 TypeRef/descriptor spellings where unsigned-ness genuinely has no vocabulary, and the V1 self-check accepts both classes
- library builds declaring sidecar integer slots force-lower the designated init/update/subscriptions exports and every model-first contract helper, so record-field and msg-arm writes enter the prove-or-refuse boundary check instead of attesting from dead-stripped vacuity
- declared record-field reads seed their class shape (whole, class range), the assumption side of the slot contract: an unguarded counter increment now refuses RANGE with the teaching triple rather than a spurious may-be-NaN wholeness complaint
- profile-export param/return classes stay out of integer_slots by design — the attestation mirrors the document's own i64-spelled slots (the V10 bijection; abi.exports carries no TypeRefs) while an export's class is attested by the artifact's C signature under the same gate; the unconsumed-path refusal teaches this
- regressions added: u64 attests u64 while the TypeRef spells i64, unproven update writes into a declared model field refuse SC4022/SC4023 by slot path, and guarded writes prove and attest