manatee: Cap ZFS ARC at 4 GiB

hosts/manatee/disk-config.nix

@@ -86,6 +86,9 @@
 
     services.zfs.autoScrub.enable = true;
 
+    # Cap the ARC at 4 GiB (default is ~all of RAM); leaves room for services on this low-memory host.
+    boot.kernelParams = [ "zfs.zfs_arc_max=${toString (4 * 1024 * 1024 * 1024)}" ];
+
     # Don't force-import the pool if it appears in use elsewhere; safer default in 26.11+.
     boot.zfs.forceImportRoot = false;