Validates the source section of MishkaGervaz form DSL.
Ensures every required action (create, update, read) is defined
either on the resource or inherited from the domain — compilation fails
when both are nil. Resource overrides win when both are set.
The verifier short-circuits when no fields are declared (a form without fields has no source to consume).
The master_check requirement is enforced upstream by
MishkaGervaz.Form.Transformers.MergeDefaults.merge_master_check_default/1,
which always persists a fallback MFA when neither resource nor domain
defines one — the verifier no longer guards that path.
See MishkaGervaz.Form.Dsl.Source,
MishkaGervaz.Form.Transformers.MergeDefaults,
MishkaGervaz.Form.Verifiers.Helpers, and sibling verifiers.