Function walk_scan 

pub fn walk_scan(
    roots: &[String],
    excludes: &[String],
) -> (VolumeIndex, ScanStats)

Walk roots (absolute base paths), pruning any subtree whose folded path matches an entry in excludes (ADR-0025), and build a queryable VolumeIndex.

Infallible by design (don’t crash): an unreadable root or directory is counted in ScanStats::walk_read_errors and skipped, never propagated. The worker maps that count to a counter + warn at its single mapping point.