Add disk cache helpers that store and retrieve sidecar payloads with
the same cache key and vary key as the HTTP response entry. The first
consumer is JavaScript bytecode.
Delete sidecars when the owning entry is removed, evicted, or replaced
by a fresh response. Count their on-disk size toward the cache budget so
bytecode data cannot grow outside eviction accounting.
Disk cache tests cover sidecar round-trips, replacement cleanup, and
eviction size accounting.
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