Clients resolve base_url and shards_base_url against the proxy's URL. A
scheme or host sends them around the proxy, and a leading slash leaves
its mount, so neither layout is served.
The shard index (repodata_shards.msgpack.zst) and the shards of CEP 16
fell through to the uncached passthrough, so clients that use them, such
as pixi, got no cooldown filtering and no offline copy.
Without cooldown, the index and shards are now served through the
metadata cache. With cooldown, shards have to be filtered, but clients
cache them by the hash in the index and never fetch a hash twice. The
proxy rewrites the index instead: each hash is XORed with a mask derived
from channel, subdir and the current hour, and shards_base_url points at
cooldown-shards/{hour}/. That route undoes the mask, fetches the
upstream shard and drops records still in cooldown. The unmasked shard
paths answer 404 while cooldown is on.
When the index puts packages or shards somewhere the proxy doesn't
route, or has an unknown version, the proxy answers 404 and clients fall
back to repodata.json, which is filtered as before.
Fixes#529
