v6.25.0

Fixed

• Concurrent fleche run --bg submissions from one shell no longer race on
  the shared SSH ControlMaster socket. Previously all but one invocation
  failed at the rsync step ([2/4] Syncing project code) because they raced
  to create the master before it was accepting connections. Master creation is
  now serialized with a per-host file lock held only around the directory
  warm-up, so the data transfers still run in parallel. The lock is acquired
  off the async runtime and is a no-op on non-Unix platforms (which don't use
  ControlMaster).
• rsync failures now surface the real SSH error instead of the OpenSSH_x.y
  verbose banner, which previously masked the underlying cause.