### `wai.image.jpegai` now dispatches to the byte-exact decoder

- `decode_envelope` prefers the deterministic `jpegai_decode::decode` when the
  model directory ships the integer weight bundle (`weights/*.json` + `norm.json`),
  falling back to the ONNX-runtime decode otherwise — existing bundles keep
  working. `wai.image.jpegai` is now registered in SPEC §5 (it was a live
  capability missing from the normative menu).
- The cross-arch byte-equality matrix now covers the merged decoder's entropy
  stage (`byte-exact-conformance`), and that crate's build — broken on a moved
  `rans.rs` include — is restored.

