catalog(chatterbox-fast): add streaming TTS service entry (streamable:true)

Land chatterbox-fast in the asset-engine catalog as an additive service, per
asset-engine-dev's shipped streaming-audition path (asset-engine v0.1.17-19):
- streamable:true -> UI routes Generate to an ephemeral progressive-<audio>
  audition (no Job/Asset); re-run on `chatterbox` to keep output.
- New service-level `streamable` bool added to services.schema.json (additive,
  default false; mirrors the Pydantic model asset-engine-dev regenerates).
- Fields: text, voice (select via /voices), temperature/top_p/top_k/
  repetition_penalty/seed, format (pcm default; UI forces wav). exaggeration/
  cfg_weight omitted — Turbo ignores them.
- status experimental until the first real browser audition verifies progressive
  playback (the one thing asset-engine-dev couldn't machine-verify).
- reproducibility + audit entries added. No catalog_version bump (additive).
Validates against the updated schema.

docs/asset-engine/services.schema.json

@@ -589,6 +589,11 @@
           "title": "Content Type",
           "type": "string"
         },
+        "streamable": {
+          "default": false,
+          "title": "Streamable",
+          "type": "boolean"
+        },
         "model": {
           "$ref": "#/$defs/CatalogModel"
         },