feat: support channel in CLI cut command

[upils]

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Allow selecting a channel when cutting slices from store packages (bins), via a pkg_slice@channel suffix on the command line.

Unlike debs, bins are published per channel, so users need to pick which one to fetch; a bare @3.0 is treated as a track and gets the default risk (stable), while @3.0/edge is used as given. When no suffix is provided, the channel is derived from the package's default-track in the slice definition, which stays a track — the risk remains implicit on the YAML side. The channel is kept strictly separate from slice identity: it flows only from the reference through setup.Select into Selection.Channels for the slicer to consume at fetch time, leaving dependency resolution, conflict checks and the manifest untouched. Conflicting channels for slices of the same package and channels on non-store packages are rejected, info/find commands reject the @ suffix since it is only meaningful when cutting, and channel validation is intentionally loose so longer forms such as track/risk/branch are not rejected today.

NOTE: This PR only prepares a Channels map in the Selection. Subsequent work to fetch bins from the store will consume it to get the right revision.

[niemeyer]

What's the background of this feature? We only promise compatibility inside a given track, so it feels problematic to offer a wide selection of tracks without also accompanying them by actual definitions.

This needs a careful spec, I believe.