httpcache: write <path>.meta JSON sidecar after each successful download;
load it on first Fetch so conditional GETs work after process restarts.
Tests verify: download, sidecar written, same-cacher 304, fresh-cacher 304
(the last being the key case — no in-memory state, sidecar drives ETag).
MaxMind integration test reads GeoIP.conf, downloads City+ASN, verifies
fresh-cacher conditional GET skips re-download via sidecar ETag.
