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/services.schema.json

@@ -218,6 +218,18 @@
           ],
           "default": null,
           "title": "Accepted Types"
+        },
+        "section": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "title": "Section"
         }
       },
       "required": [
@@ -425,6 +437,38 @@
       "title": "CatalogResponse",
       "type": "object"
     },
+    "CatalogSectionGroup": {
+      "additionalProperties": false,
+      "description": "A named group of fields, for progressive disclosure in form rendering.\n\nServices with many fields (ace-step has 27) declare section_groups and\ntag each field with a `section: <id>`. Consumers render fields grouped\nunder tabs / accordions / etc. Fields without a section go into an\nimplicit 'main' group rendered first.",
+      "properties": {
+        "id": {
+          "title": "Id",
+          "type": "string"
+        },
+        "label": {
+          "title": "Label",
+          "type": "string"
+        },
+        "hint": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "title": "Hint"
+        }
+      },
+      "required": [
+        "id",
+        "label"
+      ],
+      "title": "CatalogSectionGroup",
+      "type": "object"
+    },
     "CatalogService": {
       "additionalProperties": false,
       "properties": {
@@ -502,6 +546,13 @@
           "title": "Fields",
           "type": "array"
         },
+        "section_groups": {
+          "items": {
+            "$ref": "#/$defs/CatalogSectionGroup"
+          },
+          "title": "Section Groups",
+          "type": "array"
+        },
         "response": {
           "$ref": "#/$defs/CatalogResponse"
         },