Every fetch of an artifact wrote one path, so fetches with different URLs
or digests, which do not share a coalescing key, could overwrite each
other's bytes, serve the wrong ones, or delete them on a digest mismatch
while the other was about to open them.
storeArtifact now writes {ecosystem}/{name}/{version}/{fetch id}/{filename},
with a random id per fetch. Most ecosystems learn the digest only after
the fetch, and storage has no rename, so the id is the one rule that fits
all of them. Existing records keep their paths and stay readable. On a
file:// bucket, Delete now removes an emptied fetch directory, which
fileblob leaves behind, and no other directory, since another fetch may
be creating its own inside it.
A path a record stops pointing at is queued in pending_deletes rather
than deleted, since a request that read the record may still open it. A
loop deletes queued paths after max(1h, direct_serve_ttl), whether or not
max_size is set. UpsertArtifact and clears apply only if the record still
holds the path the caller read, so racing commits each queue the path
they replaced and a clear never orphans a newer commit. Eviction still
deletes inline, and counts space as freed only when the record still
pointed at what it deleted.
