restic-backup: note rustic as format-compatible escape hatch (lock-free prune, lower RAM)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ks4/restic-backup.md

@@ -111,6 +111,18 @@ Cache on tmpfs was considered and rejected: the cache exists to avoid
 full re-download), eats the RAM the ARC needs, and buys nothing —
 disk-backed cache reads are ARC-warm after first touch anyway.
 
+### Escape hatch: rustic
+
+[rustic](https://github.com/rustic-rs/rustic) speaks the **same
+repository format** — switching later is a binary swap on the same
+repos, no re-seed. It would solve two specific pains if they
+materialize: **lock-free prune** (no backup/prune scheduling
+constraint) and a **lower RAM footprint** (the seafile-index
+concern). Not chosen first because it isn't in Debian (manual
+upgrades) and has a far smaller community than restic. Don't run
+both tools concurrently on one repo (rustic ignores restic's locks);
+a clean switchover is fine.
+
 ## 3. Install + init (root on ks4)
 
 ```sh