Explicit host-owned catalog cache, the counterpart of the pinned client's CachePath. Shared code never writes files.

Entries are index and products JSON bodies named by their file name inside a namespace, the hex SHA-256 of the stream URL, as the pinned cache directory lays them out. read/3 returns the body and its age in milliseconds; the library applies the expiry (one hour unless cache_expiry: says otherwise). A fresh entry is used without a request; a failed request or non-200 reply falls back to any entry, expired or not; a fresh 200 body replaces it; and a 404 while downloading an image file invalidates the namespace.

Failures are ignored as natively: an error or exception from read/3 is a miss and one from write/4 or invalidate/2 leaves the result unchanged. Callbacks run in the bounded request worker. Fixture success is not installed cache acceptance.