feat: add capability_id to PackageRequest and Package (v2 format-selection at create_media_buy)

[bokelley]

Closes #4842

Summary

AdCP's v2 format-selection path (capability_id) was already wired at discovery (ProductFormatDeclaration via get_products) and fulfillment (creative-manifest, creative-asset via sync_creatives), but the buy-creation step (PackageRequest / create_media_buy) was stuck on the v1 format_ids[] vocabulary. Buyers had to translate back through v1_format_ref[] — the exact bridge this PR removes.

Changes:

• media-buy/package-request.json — adds capability_id?: string as the v2 alternative to format_ids[]. Full normative resolution rules (4 states: capability_id-only, both, v1-only, neither) and UNSUPPORTED_FEATURE error-on-miss behavior in the field description.
• core/package.json — adds capability_id?: string on the confirmed Package response; sellers echo it when the buyer used the v2 path.
• core/x-entity-types.json — registers new entity type product_format_declaration and annotates all four capability_id fields across the full storyboard (creative-manifest, creative-asset, package-request, core/package) with x-entity: "product_format_declaration".
• core/creative-manifest.json, core/creative-asset.json — adds x-entity: "product_format_declaration" to existing capability_id fields (these were un-annotated; fixed as part of the same storyboard consistency pass).
• docs/media-buy/task-reference/create_media_buy.mdx — adds capability_id row to the Package Object table, † footnote, and a new "v2 format selection" subsection with resolution-rule table and example. Updates stale normative text ("format IDs are required" → "format specification tells the publisher…").
• .changeset/package-request-capability-id.md — minor bump (additive optional field on published schema surface).

Non-breaking justification

package-request.json has "additionalProperties": true at the root. Old sellers ignore capability_id and continue processing format_ids[] unchanged. No existing required field was modified. The v1 format_ids[]-only path is explicitly preserved in all four resolution-rule states.

Pre-PR review

Two expert passes completed (2-iteration cap per playbook).

Pass 1 blockers (all fixed before push):

• x-entity annotation missing on capability_id → added product_format_declaration to registry; annotated all four cross-storyboard capability_id fields
• Error code UNSUPPORTED_FEATURE (not VALIDATION_ERROR) — aligned with product-format-declaration.json closed-set norm
• Real company names in doc examples → replaced with fictional names (streamhaus_homepage_takeover_premium, pinnacle_reels_vertical)
• Stale normative text ("Format IDs are required") → fixed at both sites in create_media_buy.mdx

Pass 2 (post-fix):

• ad-tech-protocol-expert: BLOCKED — capability_id on creative-manifest.json and creative-asset.json missing x-entity annotation → fixed in the same commit
• code-reviewer: APPROVED

Final compliance build: 244 x-entity annotations, 30/32 entity types in use. ✅

---

Opened by the AdCP triage agent · session

---

Generated by Claude Code

[bokelley]

Closing as superseded by current main. The v2 create_media_buy format-selection path has landed as capability_ids[] on PackageRequest, with corresponding create_media_buy docs and UNSUPPORTED_FEATURE guidance. This draft uses the older singular capability_id shape and would roll back the current array design.