Preserve existing managed HTTP consumer contracts

Trusted Server's managed HTTP migration needs the existing Fastly surrogate-key cache metadata and a non-consuming readiness wait. A wire header cannot replace cache metadata; consuming collect futures cannot preserve the original handles for a timer-first collection snapshot.

Keep this limited to existing mechanisms above #275:

• Add a mutually exclusive tagged platform-default cache selection, mapped to the pinned Fastly SDK setter without TTL/stale overrides. Keep other adapters' default/no-op behavior and reject tagged Image Optimizer requests before I/O.
• Expose the provider's existing paired OutboundTime and Pending's existing progress/cache path with the caller's waker. Retain is_ready's non-driving snapshot, original completion ownership and cancellation.
• Add actual SDK mapping, public API, wakeup, owner and cross-target regressions. Do not add a second transport, completion ledger, timer policy or detached task.

This is a technical compatibility task. Hosted cache fill/purge and the application's timer-first cutoff composition remain separate acceptance boundaries. No deployment or #275 rewrite is required.