ISourceFetcher over the yt-dlp binary. One process per job; the file lands in
a per-job directory so two concurrent jobs on the same URL cannot collide.
Filesystem work goes through hive-system: IPathQuery for existence,
IFilesystem for the directory it writes into.

vtranslate fetch addon facade.

Mirrors the Hive IAddon shape without making vtranslate-fetch require hive-mcp
at compile time. When hive-mcp is on the classpath, VTranslateFetchAddon is
extended to hive-mcp.addons.protocol/IAddon at runtime and init-as-addon!
returns the real IAddon record; otherwise init-as-addon! performs provider
registration only.

How to ask yt-dlp for a file, and how to read what it answers. Pure — no
process, no filesystem.
  command    - opts -> argv vector
  interpret  - a finished invocation -> Result<{:local-path .. :title ..}>

Register this addon's fetch providers on the engine's registry. Requiring this
ns is what makes [:providers :fetcher] :yt-dlp resolvable.