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/services.schema.json

@@ -332,6 +332,7 @@
     },
     "CatalogResponse": {
       "additionalProperties": false,
+      "description": "How to interpret the inference response.\n\nThe `type` is the *renderer dispatch* \u2014 what kind of asset the user\nultimately sees (audio player, text block, etc). The other fields\ndescribe how to *extract* that asset from the wire shape:\n\n  - Raw-bytes wire (e.g. /v1/audio/speech returns raw audio):\n        type: audio\n        mime: audio/wav             (or mime_from_field for dynamic)\n\n  - JSON-envelope wire with base64-encoded asset (e.g. captioned\n    speech returns {audio: <b64>, audio_format: ..., timestamps: ...}):\n        type: audio\n        audio_field: audio          # JSON key with the base64 bytes\n        audio_format_field: audio_format    # JSON key with the MIME\n        timestamps_field: timestamps         # optional structured aside\n\n  - JSON-envelope wire with text payload (e.g. ASR transcript):\n        type: text\n        output_field: text          # JSON key with the text body\n        timestamps_field: segments   # optional, for ASR-with-timestamps\n\nRules:\n  - When audio_field is set, the renderer expects a JSON envelope and\n    will base64-decode that key. The wire MIME is application/json\n    regardless of `mime`; `mime_from_field` is incompatible.\n  - timestamps_field is independent of type \u2014 any service that\n    emits structured timestamps alongside its primary output can\n    declare it; the renderer extends accordingly.",
       "properties": {
         "type": {
           "enum": [
@@ -380,6 +381,42 @@
           ],
           "default": null,
           "title": "Output Field"
+        },
+        "audio_field": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "title": "Audio Field"
+        },
+        "audio_format_field": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "title": "Audio Format Field"
+        },
+        "timestamps_field": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "title": "Timestamps Field"
         }
       },
       "required": [