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.yaml

@@ -111,11 +111,12 @@ services:
     name: Kokoro Captioned Speech
     description: >
       Kokoro TTS with word-level timestamps returned alongside the audio.
-      For subtitle generation and video sync. Same model as `kokoro`; this
-      is a separate catalog entry because the response shape is structured
-      JSON (audio inline + timestamps), not raw audio bytes.
+      For subtitle generation and video sync. Same model as `kokoro`;
+      separate catalog entry because the wire shape is a JSON envelope
+      carrying base64-encoded audio plus a structured timestamps array.
     category: tts
     version: 1
+    status: experimental
     host: irv-ml1
     endpoint: http://10.100.79.3:8193/dev/captioned_speech
     method: POST
@@ -153,32 +154,43 @@ services:
         label: Language code
         required: false
     response:
-      type: json
-      mime: application/json
+      # Stays in the closed type vocabulary: from a renderer-dispatch
+      # standpoint this IS audio. The audio_field/audio_format_field/
+      # timestamps_field decomposition tells consumers how to extract
+      # those parts from the JSON envelope wire shape — added to the
+      # catalog schema in 2026-05 specifically to support response
+      # shapes like this one without extending the type vocab.
+      type: audio
+      audio_field: audio
+      audio_format_field: audio_format
+      timestamps_field: timestamps
     reproducibility:
       seedable: false
       deterministic: true
       notes: >
-        Same determinism story as kokoro proper. Response shape (verified
-        2026-05-11 against live API):
+        Same determinism story as kokoro proper. Verified wire shape
+        (2026-05-11 against live API):
           {
             "audio": "<base64-encoded bytes in response_format>",
             "audio_format": "audio/wav" (or matching response_format),
             "timestamps": [{"word": str, "start_time": float, "end_time": float}, ...]
           }
-        Consumer must base64-decode `audio` to play; `timestamps` drives
-        subtitle/karaoke UI. Audio_format string in the payload is
-        authoritative for the decoded bytes.
+        Consumer base64-decodes `audio` to play; `timestamps` drives
+        subtitle/karaoke UI. The response decomposition fields above
+        encode this so the renderer doesn't need per-service-id branches.
     estimated_latency:
       cold_start_s: 2
       warm_per_unit: "~same as kokoro proper, plus minor overhead for timestamp emission"
     license: Apache-2.0
     notes: |
-      `return_timestamps` and `stream` upstream params are deliberately
+      `return_timestamps` and `stream` upstream params deliberately
       omitted from the catalog: timestamps must be on for this endpoint
       to be meaningful, and streaming + JSON-with-base64 don't compose.
-      `download_format` / `return_download_link` skipped — same reasoning
-      as kokoro proper.
+      `download_format` / `return_download_link` skipped — same as kokoro
+      proper.
+
+      status: experimental until the consumer's audio-with-timestamps
+      renderer ships. Once present, flip to status: ready.
 
   - id: chatterbox
     name: Chatterbox Turbo TTS