### JPEG AI — a live, self-sufficient `wai.image.jpegai` decode (2026-06-22)

- `neural::jpegai::decode(codestream, &JpegAiModel)` decodes a real ISO/IEC 6048
  codestream to RGB with **no per-image schedule fixtures**: every per-image
  quantity is derived from the bitstream, and only checkpoint constants come
  from a sink-supplied model bundle. Wired into `decode_envelope` — JPEG AI is
  now a dispatchable capability, not just reference/conformance support.
- The two derivations that closed it, traced bit-exact from the reference: the
  residual coding mask is `sigma_skip > 382` (the σ surface *before* the rvs
  addition — on the bucketed index it looks ambiguous, on `sigma_skip` it is
  exact, 0 violations / 65536 positions); the dequant is
  `resi × rvs_dq[grfs[ch]][lk] / 2^16 / scaler_vec[ch]`, with `grfs` determined
  the same way the σ rvs does so the per-channel `scaler_vec` is consistent.
  Matches the reference decoder's PNG to within rounding (max diff 1), checked by
  `tests/jpegai_live.rs` on a machine holding the local JPEG-AI assets; no CI job
  runs it (its model bundle is not in the repository).

