Space keeps its scans and refreshes them: zfs diff, folder mtimes, resume after cancel

A scan is saved beside the config and read back after a restart. The next scan starts
from it: on a pool with snapshots zfs diff names what changed and only those folders are
read; elsewhere a folder whose mtime is unchanged keeps its list and only its kept files
are stat'ed again. A cancelled scan keeps what it walked and Resume reads the rest.
Forget drops the saved result. The treemap is every file a tile, grouped by folder, with
no nested boxes: the folder chains of a Linux system made a staircase of empty headers.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

cmd/hamlab/main.go

@@ -77,7 +77,7 @@ func main() {
 	case "fs": // privileged helper for the Files tab (run as root by the daemon)
 		os.Exit(files.Helper(flag.Args()[1:], os.Stdin, os.Stdout, os.Stderr))
 	case "space-walk": // helper for the Space tab: measure mountpoints, stream the tree back
-		os.Exit(space.Helper(flag.Args()[1:], os.Stdout, os.Stderr))
+		os.Exit(space.Helper(flag.Args()[1:], os.Stdin, os.Stdout, os.Stderr))
 	case "install", "uninstall":
 		var err error
 		if cmd == "install" {