Two monitors can keep different wallpapers. `set --output` targets one
awww output, writes that output's bread-theme files, and updates
~/.config/breadpaper/current.json without clobbering the other. Global
set still runs wal + bread-theme reload and stamps every known output.
`apply` and listen-on-monitor-connect restore the saved map. The library
picker binds to its monitor and applies to that output.