Add required-column validation to the R CLI

[eneskemalergin]

Summary

The R CLI assumes the expected Spectronaut or MSFragger columns are present. If the wrong export is provided, the script currently fails with low-level column errors.

Tasks

• define the required columns for each supported source
• validate the input table immediately after loading it
• print a clear missing-column error message

Acceptance criteria

• the R CLI fails early with a readable schema error
• the error lists the missing columns for the selected source

Why this matters

Public users should receive a clear explanation rather than raw R errors.