Replace r4.channel+track with r4 view?

[oskarrough]

Related to the discussion in #8 I'm wondering about changing the commands a bit.

• r4 view '' -> data to stdout
• r4 dl '' -> media/files to disk for tracks in that view
• r4 backup '@channel' -> snapshot bundle, basically a view + plus timestamp
• remove r4 channel + track

ViewURI is the abstraction. Reuse the one from r4-sync-tests/docs/views.md.

• r4 view '' is the main read command.
• r4 view returns one shape:
  {
  "uri": "@ko002 #house?limit=20",
  "channels": [],
  "tracks": []
  }

More notes..

• view is the export/read story
• dl is the file/materialization story
• backup is just a named snapshot format, not a core abstraction
• no separate export command
• avoid old channel view / track list --channel style
• let the URI carry query semantics instead of duplicating them as flags
  Minimal flags for view:
• --format <text|json|ndjson|sql|m3u>
• maybe --limit, --order, --direction only if you want CLI overrides on top of URI semantics, but ideally keep those in the URI
  So the UX becomes:

r4 view '@ko002'
r4 view '@ko002 #house?limit=50'
r4 view '@alice;@bob'
r4 dl '@ko002'
r4 backup '@ko002' > ko002.backup.json

Not sure about the @ symbols everywhere. bit annoying, but needed for the view string at least..