pub fn sniff_headers(src: &str) -> HeaderSniffExpand description
Best-effort detection of a document’s target version AND packaging axis
from its source text — see sniff_version’s doc comment for the
version-detection rules. The one addition: a use-shaped header line
(is_use_header) sets both version = Some(V0_1) and
envelope_headers = true, so the CLI’s detection ladder can pin
LoadMode::Envelopes off it.