Server-side ad templates design spec

[jevansnyc]

Summary

• Adds design spec for server-side ad templates, replacing client-side SDK slot discovery (Prebid.js, Smart Slots) with edge-configured creative opportunities
• TS matches URL patterns to pre-configured slot templates, fires the full auction (PBS + APS + all providers) at request receipt in parallel with origin fetch, and injects winning bids into head before the browser receives HTML
• Estimated ~2,000ms improvement in time-to-ad-visible (from ~3,200ms to ~1,200ms)

Test plan

• Review spec for accuracy against current auction pipeline
• Validate creative-opportunities.toml schema covers publisher slot inventory
• Confirm open questions with publisher (URL patterns, homepage slot count, PBS stored request setup)