orb v0.4.0-beta

The first 0.4.0 artifact that exists: the alpha's tag was published with
nothing attached to it, because a red test stage skips the appimage and
release jobs silently.

What is new since then, in one line each:

- The room, in 3D, read from photographs of the actual venue -- many images
  per read, measurements first, and the import places the fixtures.
- Beam angle, aim and gel per fixture, and a colour for a dimmer-only one.
- A DMX universe can be rebound live from a sectioned gear dialog, instead of
  editing dmx.toml and restarting.
- The assistant keeps its conversations, shows its progress while a turn runs,
  and takes a model switch in the dashboard.
- A Spotify sequence playing to its end no longer freezes the desk, along with
  four siblings of the same blocking-under-the-World-lock fault.
- A multi-pick is one sequence on one deck, named after the track.
- help, grammar-driven completion, history and Ctrl-R on the command line.
- Key, downbeat, loudness and structural segmentation in orb-dsp.
- Every orb serves its own manual at /docs/.

See CHANGELOG.md for the full list.