Integrate cache policies with runtime shared caches

[ChristianPavilonis]

Summary

Make Trusted Server cache policies effective through each runtime's actual shared-cache mechanism and move platform-specific cache rendering behind the EdgeZero adapter boundary.

PR #860 emits typed browser and runtime-specific edge-cache headers. Cloudflare Workers Cache can consume the emitted Cloudflare-CDN-Cache-Control header when enabled, but Fastly synthetic/egress responses still need explicit cache integration. Core handlers also hand-thread a platform constant that belongs at the runtime boundary.

Scope

• Fastly:
  • choose and implement Core Cache/Simple Cache for synthetic responses such as TSJS;
  • apply appropriate backend cache overrides for proxied responses;
  • verify TTL and privacy/bypass behavior.
• Cache keys:
  • prove /static/tsjs= shared-cache keys preserve the v query parameter;
  • prevent versioned immutable and missing/mismatched short-TTL responses from colliding.
• EdgeZero/runtime abstraction:
  • own platform edge-cache header names and CachePolicy rendering at the adapter boundary;
  • remove hand-threaded EdgeCacheHeader constants from Trusted Server core signatures.
• Cloudflare:
  • verify Workers Cache hit/bypass behavior;
  • upgrade compatibility_date separately with integration coverage.

Acceptance criteria

• Versioned TSJS responses are demonstrably stored and hit in each production shared cache.
• Missing/mismatched TSJS versions retain short TTLs and cannot share a key with matching versions.
• Private, no-store, and cookie-bearing responses bypass shared storage.
• Platform-specific cache rendering is owned by EdgeZero adapters rather than core handlers.
• Fastly and Cloudflare integration tests cover the runtime behavior.

References

• PR Implement configurable cache header policies #860
• docs/superpowers/specs/2026-07-06-cache-control-header-design.md
• docs/superpowers/plans/2026-07-06-cache-control-header-implementation-plan.md
• Issue Design TS dynamic response caching for origin templates and RSC/API #859 for the separate dynamic/template-cache design