compliance(sales-social): require preview_creative with catalog binding for substitution-safety coverage

[bokelley]

Why

Spun off from #2651, which assumed sales platforms have no preview surface and proposed a new dry_run_substitution capability. That premise doesn't hold — Meta DPA, TikTok, Snap, Amazon Ads, and Walmart Connect all expose preview APIs that exercise catalog-item macro substitution (Meta's creative_preview, etc.). The right move is to require the existing preview_creative capability, not invent a new one.

What

Extend sales-social required_tools (static/compliance/source/specialisms/sales-social/index.yaml:8-12) to include preview_creative, with the obligation that preview output exercises catalog-item macro substitution when a catalog binding is provided.

Hook the existing substitution-observer-runner (modes html_inline and url_fetch) to the preview_creative response — same contract creative-generative already uses post-#2649.

Scope

• sales-social only. sales-retail-media is not yet in the specialism enum; same requirement gets added when that specialism lands.
• No new capability or schema. Reuses preview_creative and the existing observer runner.

Known coverage limitation

Preview-time substitution may diverge from serve-time substitution on platforms where the two execute different code paths. This is the same gap creative-generative carries today and should be documented in the storyboard rather than gated on a new attestation mechanism. Real-impression introspection is out of scope for 3.1.0 — file a follow-up if a platform demonstrates serve-time divergence in production.

Acceptance

• preview_creative added to sales-social required_tools
• Storyboard phase added that calls preview_creative with a catalog-bound DPA template and runs the substitution-observer assertions (expect_substitution_safe)
• Spec doc notes the preview-vs-serve-time coverage limitation
• Closes the gap left by compliance: substitution-safety observation hook for specialisms without a preview surface (sales-social, sales-retail-media) #2651

Related

• spec(creative): catalog-item macro URL escaping + nested-expansion prohibition (#2558) #2620 substitution rule (merged)
• compliance: substitution-observer-runner contract for catalog-item macro safety (#2638) #2647 substitution-observer-runner (merged)
• compliance: extend expect_substitution_safe to creative-generative (#2638 follow-up) #2649 creative-generative observer extension
• compliance: substitution-safety observation hook for specialisms without a preview surface (sales-social, sales-retail-media) #2651 (closed — premise revised)