Function compute 

pub fn compute(
    root: &Path,
    lock: &LockFile,
    target: &TargetState,
    force: bool,
) -> Result<SyncDiff, MarsError>

Compute the diff between current disk state + lock and target state.

Uses dual checksums from the lock file:

• source_checksum: what the source provided
• installed_checksum: what mars wrote to disk

Compares current disk hash against lock checksums to determine the diff entry variant.