v0.5.5

Fixed

• FileBlobStore::open now takes an exclusive flock(2) on
  <data_dir>/store.lock and holds it for the lifetime of the
  instance. Two live instances on one data directory previously
  replayed manifest.log into the same next_slot, assigned the
  same slot to different blob GUIDs, and appended conflicting set
  deltas — permanently poisoning the manifest (every later open
  failed with FileBlobStore::Manifest::duplicate slot) while the
  colliding frames overwrote each other in blobs.dat. Since 0.5.0
  even read-only snapshots persist frozen root frames, so the
  overlap window of a plain handover (store = reopen(path)) was
  enough to trip this. Open now waits up to 5 s for the previous
  instance to finish dropping, so handover reopen serializes; a
  genuinely concurrent second opener fails with a clear
  WouldBlock error instead of corrupting the store. Same-process
  double-opens are caught too (flock is per open-file-description),
  and the kernel releases the lock if the holder crashes.