Add strict validation for untrusted raw step fragments

Follow-up extracted from #1061. Raw authored steps can still reach `Step::RawYaml`; add a strict validation path for isolated untrusted fragments using the current typed task parsers and lint findings. Define accepted shapes, reject unsafe or ambiguous input, and add compile/lint contract tests.

Start by tracing how authored steps reach Step::RawYaml and how the current typed task parsers produce lint findings. Define the accepted shapes for isolated untrusted fragments, reject unsafe or ambiguous input, and add compile and lint contract tests that demonstrate the validation boundary and its completed behavior.

Written by the indexing model from the issue text.