catalog-contract: add response-decomposition fields (audio_field, timestamps_field, audio_format_field)

asset_engine consumer needed to render kokoro-captioned, whose wire
shape is a JSON envelope carrying base64-encoded audio plus a
structured timestamps array. Modeling it as response.type=json
would force either a per-service-id renderer (forbidden by
brief §1.7) or extending the closed response-type vocabulary
(forbidden by brief §2.2 without a coordinated bump).

Resolution (per althing thread 01KRCF4W66X3): keep response.type
closed at the existing six values and decompose at the response
*field* level instead — the same flexibility seam already used by
mime / mime_from_field / output_field. Adds three optional keys:

  - audio_field: JSON key holding base64-encoded audio bytes
  - audio_format_field: JSON key holding the decoded audio MIME
  - timestamps_field: JSON key holding a structured timestamps array
    (independent of type, declared by any service emitting time-
    aligned markers)

Validators in CatalogResponse enforce sane combinations:
  - audio_field requires response.type=audio
  - audio_field forbids mime_from_field
  - audio_format_field requires audio_field

This is additive and backward-compatible — no catalog_version bump,
existing services parse unchanged. CATALOG-CONTRACT.md updated with
the new rows in the response-field table and a versioning-policy
row codifying that adding optional keys to response: doesn't bump.

kokoro-captioned re-shaped to use the new schema:
  response:
    type: audio
    audio_field: audio
    audio_format_field: audio_format
    timestamps_field: timestamps
And marked status: experimental until the asset_engine consumer's
audio-with-timestamps renderer ships.

JSON Schema regenerated to reflect the new Pydantic shape.

Pydantic-model side of this change lives in the asset_engine repo
at src/asset_engine/catalog.py — committed there separately.

docs/asset-engine/CATALOG-CONTRACT.md

@@ -46,10 +46,13 @@ reproducibility_audit: [Audit]     # one entry per service
 | `model.revision`  | string \| null                            | no       | SHA when known |
 | `model.image`     | string                                    | yes      | container image ref this is hosted from |
 | `fields`          | list[Field]                               | no       | request parameters; empty for catalog-deferred |
-| `response.type`   | enum: audio, image, video, text, json, file | yes    | response renderer hint |
-| `response.mime`   | string                                    | no       | static response MIME |
-| `response.mime_from_field` | string                          | no       | name of a field whose value determines the MIME |
-| `response.output_field`    | string                          | no       | for JSON responses, the key holding the asset |
+| `response.type`   | enum: audio, image, video, text, json, file | yes    | renderer dispatch (closed vocabulary) |
+| `response.mime`   | string                                    | no       | static response MIME (raw-bytes wire) |
+| `response.mime_from_field` | string                          | no       | name of a field whose value determines the MIME (raw-bytes wire) |
+| `response.output_field`    | string                          | no       | for text/json responses, JSON key holding the asset |
+| `response.audio_field`     | string                          | no       | for `type: audio` with JSON-envelope wire, JSON key holding base64-encoded audio bytes |
+| `response.audio_format_field` | string                       | no       | for `type: audio` with JSON-envelope wire, JSON key holding the decoded audio MIME (e.g. "audio/wav") |
+| `response.timestamps_field` | string                         | no       | JSON key holding a structured timestamps array — independent of type, declared by any service that emits time-aligned markers alongside its primary output |
 | `reproducibility.seedable`    | bool                          | yes      | does the endpoint accept a seed? |
 | `reproducibility.deterministic` | bool                        | yes      | same params → same bytes? |
 | `reproducibility.notes`       | string                        | no       | gotchas |
@@ -91,6 +94,7 @@ Same change-management as field types.
 |----------------------------------------------|-----------------------|
 | Add a new service                            | nothing               |
 | Add a non-required field to an existing service | service `version:` |
+| Add an optional key to the `response:` schema (e.g. audio_field, timestamps_field) | nothing — additive, backward-compatible |
 | Change a field's type, range, or default     | service `version:`    |
 | Remove a service                             | service `version:` (sentinel: removed=true), then drop in next catalog_version bump |
 | Add a new entry to the field-type vocabulary | `catalog_version:`    |