Closed public diagnostics for private profile selection and source capture.

The catalog translates internal capture reasons into stable, bounded, path-free frontend codes and messages. Unknown reasons are never promoted to public vocabulary; classify!/1 collapses them to profile_setup_failed.

This catalog is intentionally separate from PtcRunner.Kernel.DiagnosticCatalog. Kernel diagnostic rows describe command envelope phases, while these rows are emitted by the one-shot REPL frontend.