Function reachable_set 

pub fn reachable_set(
    start: &Path,
    census: &[CensusEntry],
    content_bodies: &[PathBuf],
) -> BTreeSet<PathBuf>

The set of workspace-relative paths a walk from start reaches: start itself, every path a census link resolves to (any relation, a body wikilink, or an id through the registry), and every content target.

A case-mismatched link counts its actual on-disk file as reached, so a file is never both case-mismatched and orphaned. Prose bodies (and attachment payloads) arrive through content_bodies rather than the census, because a content pointer is not a graph edge — but it does reach a file, which is what every caller here cares about.

The one definition of “reachable” that the orphan check, the fixity pass, the vocabulary pass, and the history capture set all share (DESIGN §8).