3.1: Re-add preview specialisms (streaming-tv, exchange, retail-media, measurement-verification)

[bokelley]

Context

In 3.0 these four specialisms were removed from the enum and compliance catalog to avoid ambiguity about what's GA vs preview (per community feedback during GA prep — see PR #2302). Reintroducing them in 3.1 means adding them with full storyboards, not preview flags.

Scope

Re-add these four specialism IDs with stable storyboards:

• sales-streaming-tv — Connected TV / streaming video (ad-supported) under the media_buy protocol. Channel: ctv.
• sales-exchange — Programmatic exchange / SSP exposing auction-based inventory under the media_buy protocol.
• sales-retail-media — Retail media network seller (onsite, offsite, in-store) under the media_buy protocol.
• measurement-verification — Third-party viewability, attribution, brand-safety, and SI-outcome verification. Candidate for a dedicated measurement protocol rather than parking under governance.

Work

• Add enum values back to static/schemas/source/enums/specialism.json
• Author storyboards under static/compliance/source/specialisms/{id}/
• Decide the measurement-verification parent protocol (governance vs new measurement)
• Update compliance catalog, learning specialist modules, and glossary examples
• Publish migration guidance for 3.0→3.1 (additive — no removals)

Notes

At 3.0 GA, sellers in these categories should claim sales-guaranteed or sales-non-guaranteed. They are not blocked from participating — the category-specific storyboards are just not yet authoritative.