feat: support track in CLI cut command

[upils]

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Add support for selecting a track when cutting slices from store packages (bins), via a pkg_slice@track suffix on the command line.

Unlike debs, bins are versioned by track, so users must be able to choose which track to fetch; the default-track from the slice definition is used when no suffix is provided. The track is kept strictly separate from slice identity — it flows only from the CLI reference through setup.Select into Selection.Tracks for the slicer to consume at fetch time, leaving dependency resolution, conflict checks and the manifest untouched.
Conflicting tracks for slices of the same package, and track suffixes on non-store (deb) packages, are rejected with clear errors. The info and find commands reject the @ suffix, since track selection is only meaningful when cutting.