schemas: restructure core/format.json outer assets oneOf to be discriminated

[bokelley]

Background

Follow-up from #3917 / #3928 / #3934. The outer oneOf at `core/format.json` `#/properties/assets/items` (15 variants) cannot be cleanly discriminated by a single key today.

• Variants 0–13 (`Individual{Image,Video,Audio,...}Asset`) declare `item_type: "individual"` and `asset_type: ""`
• Variant 14 (`RepeatableGroupAsset`) declares `item_type: "repeatable_group"` and has no `asset_type`

Adding `discriminator.propertyName="asset_type"` fails because variant 14 doesn't have it. Adding `discriminator.propertyName="item_type"` doesn't help downstream codegen because 14 of the 15 variants share the same `item_type` value.

Proposed restructure

Split the union into two tiers:

```json
{
"discriminator": { "propertyName": "item_type" },
"oneOf": [
{
"properties": { "item_type": { "const": "individual" } },
"required": ["item_type"],
"discriminator": { "propertyName": "asset_type" },
"oneOf": [ ...14 individual variants... ]
},
{
"properties": { "item_type": { "const": "repeatable_group" } },
"required": ["item_type"],
// RepeatableGroupAsset shape
}
]
}
```

This produces a TypeScript codegen output along the lines of:

```ts
type Item =
| (IndividualAssetCommon & ({ asset_type: 'image'; ... } | { asset_type: 'video'; ... } | ...))
| RepeatableGroupAsset;
```

instead of today's flat 15-way structurally-overlapping union.

Acceptance

• `core/format.json` outer oneOf classifies as ✓ in `scripts/audit-oneof.mjs` without the variant-14-misses-asset_type error
• `npm run test:json-schema`, `test:schemas`, `test:composed` all pass
• `scripts/oneof-discriminators.baseline.json` ratchets to remove the outer oneOf entry
• adcp-client / adcp-py codegen for `Format.assets[]` produces a discriminated union

Refs

• spec(adcp): discriminator audit — every oneOf needs a discriminator key + CI assertion #3917 — discriminator audit issue
• spec(schemas): add discriminator hints to const-property oneOf unions (adcp#3917) #3928 — first wave (const-property hoists)
• spec(schemas): hoist pricing-option + format inner-asset discriminators (adcp#3917) #3934 — second wave (cross-file refs and inner format oneOf)

[bokelley]

Triage

Classification: Schema restructure (feature)
Bucket(s): spec / protocol, schema
Status: deferred

Blocked-on: #3934 — resurfaces on merge.

PR #3934 (open, same author, same file) explicitly deferred this outer 15-variant oneOf restructure and noted "Will file an issue." This issue is the tracker for that remaining work. Once #3934 merges, triage will resurface it for the two-tier discriminator restructure + codegen follow-up (adcp-client / adcp-py).

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Triaged by Claude Code. Session: https://claude.ai/code/session_01YUMTQJUrnAPF5WMN5FwAq2

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Generated by Claude Code

[bokelley]

Moving to 3.2.0 — restructuring core/format.json outer assets oneOf. Part of the discriminator cluster (#3917/#3936/#3939). 3.2.

[bokelley]

/triage execute

Implement the complete discriminator restructure against all current variants, preserving accepted wire payloads. Ratchet the oneOf audit baseline and verify both TypeScript and Python codegen plus schema/composed tests; do not stop at schema prose.