feat(media-buy): add per-product list application receipts

[bokelley]

Summary

• add a shared list_applications receipt to legacy and canonical product responses
• report property and collection list effects symmetrically at per-product transaction-response grain
• bind receipts to list identity, seller snapshot/evaluation timestamps, and matched/unmatched summary counts without echoing fetch credentials
• document discovery, proposal, refinement, field-projection, legacy property_list, and zero-match inclusion/exclusion semantics

Why

The collection walkthrough describes seller-specific match counts, but those counts vary by product and response. The working group agreed that the seller product response—not a separate governance-agent reporting task—is the correct surface. This also keeps property and collection governance aligned and leaves fuzzy content-standards calibration as a separate concern.

Compatibility

This is additive: the new product fields are optional in the schemas and existing governance list-response schemas are unchanged. When an effective list is present, the protocol requires the seller to include the receipt regardless of field projection.

Validation

• npm run test:targeting-aware-discovery
• npm run test:schemas
• npm run test:oneof-discriminators
• npm run check:registry
• npm run test:docs-nav
• npm run test:owned-links
• npm run lint:schema-links
• npm run build:compliance -- --check
• node scripts/check-changeset-protocol-scope.cjs origin/main
• npx --yes @changesets/cli@^3.0.0 status --since=origin/main

Closes #6736

[bokelley]

@lukasz-pubx The working group aligned on putting list-match attribution on each returned product, rather than adding a seller-to-governance reporting task. This PR implements that direction across legacy and canonical product discovery, with property/collection parity. CI is green—would you please review?

[aao-secretariat]

Ladon verdict: Escalate to human review

Escalate — gated schema paths require human/CODEOWNERS review.

This PR modifies protocol source-of-truth files under static/schemas/source/** (a deterministic hard approval gate), and review_decision is REVIEW_REQUIRED, not APPROVED. Per decision-table row 2, the outcome must be escalate until a real human/CODEOWNERS approval satisfies the gate.

The reviewer found no Critical/High/Medium findings — the change is an additive, product-scoped list_applications receipts feature with a properly discriminated oneOf (const per branch, baseline untouched), closed shape enforcing the MUST-NOT-echo-auth_token rule, coherent schema↔docs, symmetric fields across legacy/canonical product schemas, and a correct minor changeset. The diff is mechanically clean, but the gate is non-negotiable absent APPROVED status.

Escalation reasons

• Touches gated paths static/schemas/source/** (canonical-product.json, inventory-list-application.json [added], product.json, index.json, get-products-request.json, get-products-response.json, product-fields.json, refine-proposals-response.json) with review_decision: REVIEW_REQUIRED — human/CODEOWNERS review required to satisfy the hard approval gate.

Why human review

• Modifies gated protocol-source files under static/schemas/source/** (canonical-product.json, inventory-list-application.json added, product.json, index.json, get-products-request.json, get-products-response.json, product-fields.json, refine-proposals-response.json) while review_decision is REVIEW_REQUIRED — human/CODEOWNERS approval required to satisfy the hard approval gate.
• This PR touches a path under a hard, non-overridable approval gate (static/schemas/source/core/canonical-product.json (modified) matches static/schemas/source/**; static/schemas/source/core/inventory-list-application.json (added) matches static/schemas/source/**; static/schemas/source/core/product.json (modified) matches static/schemas/source/**; static/schemas/source/index.json (modified) matches static/schemas/source/**; static/schemas/source/media-buy/get-products-request.json (modified) matches static/schemas/source/**; static/schemas/source/media-buy/get-products-response.json (modified) matches static/schemas/source/**; static/schemas/source/media-buy/product-fields.json (modified) matches static/schemas/source/**; static/schemas/source/media-buy/refine-proposals-response.json (modified) matches static/schemas/source/**) and the current GitHub review decision is 'REVIEW_REQUIRED', not APPROVED. This is a hard gate enforced in code — Ladon cannot auto-approve until a human/CODEOWNERS approval is recorded, regardless of how clean the diff is.