feat: add validate-release debug command

[upils]

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This branch adds a new chisel debug validate-release command that performs static validation of a Chisel release — checking chisel.yaml and the slice definition files for structural issues (path conflicts, essential loops, invalid prefer relationships, archive priority conflicts, etc.) without downloading any package.

Previously, validating a release could only be done indirectly by invoking a command that performs substantial additional work (cut builds a rootfs, find/info query slices, debug check-release-archives downloads .deb packages and inspects tarball contents). There was no lightweight, offline-capable entry point focused solely on validation. This is awkward for slice definition authors iterating locally, for CI on chisel-releases PRs, and for reviewers wanting quick feedback — all of whom benefit from a fast, credential-free check that surfaces the descriptive errors from validate() directly.