One immutable file per key, published by rename(2) inside the store. Two jobs
publishing the same key both produce a valid entry and the last wins; a reader
holding the previous one keeps reading it. No advisory locking, which is the
property a VMware shared folder cannot offer with several jobs in flight --
the objection raised in the furyrs thread.
The key joins its parts with NUL so ("ab","c") and ("a","bc") cannot collide,
and carries a format version so the whole store can be invalidated at once
when the prune rules change and older entries stop being trustworthy.
The feature set is deliberately absent from the key: cargo already encodes it
in each artifact's own hash suffix, so several feature graphs coexist in one
entry rather than fragmenting the store into one entry per variant.
