A PUT with HTTP preconditions took the per-object namespace write lock
before ingesting the request body and held it until commit, so any
concurrent read of the same object queued behind client-paced body
ingestion until the 5s acquire timeout and surfaced as 503. Exposed as
a deterministic S3 Implemented Tests gate failure when #6770 routed
1 MB conditional writes onto the streaming path (rustfs/backlog#2074).
Keep a lock-free advisory precondition check before the body for fast
412/404, and evaluate the authoritative check under the put_object
commit lock, reusing the deferred shape data movement already uses.
Reads during ingestion now return the last committed version, and a
precondition invalidated mid-stream fails closed with 412 at commit.
