The subset of the platform's Cache this package calls.
A real caches.default satisfies it structurally.
Scope is per-datacenter. caches.default is the colo's own cache: a hit
rate measured in one location says nothing about another, and a delete does
not evict globally. It is an edge-latency optimisation, not a shared store —
for that, use CAPABILITIES.CACHE over KV.
Stores a response.
Throws on the platform's own refusals — a non-GET request, status 206,
Vary: *, or an uncleared Set-Cookie — which is why
assessCacheability checks first rather than letting this reject.