fix(storage-guard): remove '!' from ConditionPathIsMountPoint

CRITICAL BUG FIX caught by live test v6 on sapphira.

The '!' prefix INVERTS the systemd test:
  ConditionPathIsMountPoint=!/path  → test passes if path is NOT a mount
                                     → unit STARTS when storage is unmounted
                                     → exactly the opposite of what we want

Correct semantics for a storage guard:
  ConditionPathIsMountPoint=/path   → test passes if path IS a mount
                                     → unit starts ONLY when storage is mounted
                                     → unit refuses to start when storage is gone

With the inverted condition, postgresql started on empty bind-mount
after lazy-umount of /home/oqyude/External — the exact silent-data-loss
scenario R1.2 is supposed to prevent.

This is the canonical 'bug the test caught' case. Live test v6 on
sapphira (2026-10-10) demonstrated: with '!' the guard does nothing,
without '!' the guard fires correctly.

Lesson: always run a live test of the guard, don't trust nix eval alone
for systemd Condition* semantics — they're evaluated by systemd at
runtime, and '!' inverts the test.

lib/xlib/helpers.nix

@@ -179,6 +179,6 @@ in
   #   serviceConfig = xlib.helpers.mkStorageGuard xlib // { ...other fields... };
   mkStorageGuard = xlib: {
     RequiresMountsFor = [ xlib.dirs.server-home ];
-    ConditionPathIsMountPoint = [ "!${xlib.dirs.server-home}" ];
+    ConditionPathIsMountPoint = [ xlib.dirs.server-home ];
   };
 }