catalog-contract: section_groups + Field.section for progressive disclosure; ace-step v2

asset_engine consumer (althing thread 01KRCJF7NGMXYE9F62Q1A6KFD4)
needed structure for ace-step's 27-field form. Two additive Pydantic
changes — backward-compatible, no catalog_version bump per the
policy table:

  - CatalogField.section: str | None = None
  - CatalogService.section_groups: list[CatalogSectionGroup] = []
  - new CatalogSectionGroup model: {id, label, hint?}

Validator: every Field.section value must reference a declared
section_groups[].id within the same service; section_groups[].id
values are unique. CATALOG-CONTRACT.md updated with both the new
service-fields row and a versioning-policy row covering
"add optional Field/Service keys -> no bump."

ace-step entry rewritten to use the new schema:
  - bumped version 1 -> 2
  - declared 6 section groups (basic / generation / conditioning /
    a2a / lora / output) with hints
  - tagged every field with a section
  - added previously-missing checkpoint_path (required: true,
    default: "/app/checkpoints" — the container's mount path).
    Wrapper-side cleanup (default in infer-api.py) queued as
    follow-up.
  - changed lyrics from optional: true -> required: true with
    default "" to match upstream's `lyrics: str` shape (empty
    string satisfies it).

JSON Schema regenerated.

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

docs/asset-engine/CATALOG-CONTRACT.md

@@ -46,6 +46,7 @@ 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 |
+| `section_groups`  | list[SectionGroup]                        | no       | named groups for progressive disclosure; consumers render fields under tabs/accordions per group. SectionGroup = `{id, label, hint?}`. Field.section refs must resolve here. |
 | `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) |
@@ -95,6 +96,7 @@ 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 |
+| Add an optional key to a Field (e.g. section) or to Service (e.g. section_groups) | 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:`    |