Add a `validate` option

Ideally cps-config would always reject straight up invalid constructs, it would also be useful to have an option to validate a CPS file, both as a sort of linter, as well as to provide warnings that might not prevent a CPS file from running, but leave out expected data

The issue names no files, tests, or entry points. Start by mapping the existing CPS parsing and command-line behavior, then clarify the validation rules, warning policy, and expected interface before estimating implementation. Done means a defined validate option with agreed diagnostics and coverage for invalid and incomplete CPS data.

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