Re-downloading a model that was already in progress spawned a second
download_worker. Both contend for huggingface_hub's per-blob file lock —
the first downloads, the second blocks on the lock and reports 0% forever
("Downloading full repository…"). Two causes, both fixed:

- Same-process re-download click: api_download_model now dedups via
  _active_download_session(model_id, file_pattern) and attaches the client
  to the live session instead of spawning a rival worker.
- Restart case: workers were plain Popen children with no parent-death
  signal, so a server/engine restart orphaned them (still holding the lock)
  while the new instance lost its in-memory dedup state. Workers now spawn
  with PR_SET_PDEATHSIG=SIGKILL so they die with the server; the re-download
  then resumes cleanly from the .incomplete blob.

Also render engine "Loading weights" tqdm progress as a single updating
line on a TTY (in-place \r) and throttle to whole-percent changes when
piped, instead of one line per update.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>