Function build_document 

pub fn build_document(
    manifest: &PatchManifest,
    applied: &[String],
    opts: &BuildOptions,
) -> Option<Document>

Build a VEX document from a manifest and a set of applied PURLs.

applied is a list of PURLs that have been verified (or were declared verified via --no-verify). Manifest entries not in applied are silently dropped — see the design note in vex::verify for why we never emit affected.

Returns None when no statements can be emitted (no applied patches matched the manifest). The CLI converts None into a non-zero exit code per the agreed contract.