Function apply_retention_preserving 

pub fn apply_retention_preserving(
    workspace: &Path,
    policy: RetentionPolicy,
    preserve_session_id: Option<&str>,
) -> Result<usize, SessionStoreError>

Apply retention while preserving one session directory, even when its existing manifest still says completed (for example, a resumed session).

The preserved path is resolved with the same session-id sanitization as the canonical store and is compared against the direct child discovered on disk. It is never taken from a manifest.