Function detect_source_path_fork 

pub fn detect_source_path_fork(
    old: &SnapshotPolicySpec,
    new: &SnapshotPolicySpec,
    has_history: bool,
    acknowledged: bool,
) -> Option<ValidationError>

Pure decision for the fork-on-edit guard on a per-source path change. A PVC’s kopia source path is part of its identity, so changing sourcePathOverride on a PVC that already has history orphans that PVC’s snapshots exactly as a username/hostname change would. Sources are matched across the edit by PVC name (paths are never CEL-driven, so an old-vs-new spec diff is complete); selector/NFS sources are out of scope. Returns the first offending change.