Fresh Live USBs don't report an ownership update (#840)

The library-ownership migration compared the owner of
/mnt/persistence/wrolpi after creating it, so on a new persistence
partition the root-owned directory always looked like an old library and
the first-boot dialog said "Updated the library's ownership for this
WROLPi version".  Only re-own a library that already existed.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

debian-live-config/config/includes.chroot/usr/local/sbin/wrolpi-bootstrap.sh

@@ -175,15 +175,16 @@ if [ "$MODE" = "direct" ]; then
   # script (first boot) or live-boot's initramfs (subsequent boots).
   mkdir -p /mnt/persistence
   mountpoint -q /mnt/persistence || mount "$PERSIST_PART" /mnt/persistence
-  mkdir -p /mnt/persistence/wrolpi
   # The library must be owned by this image's wrolpi.  ISOs before wrolpi
   # became a system account wrote it as UID 1000, which a USB upgraded with a
-  # newer ISO would leave unwritable; re-own it once.
-  if [ "$(stat -c %u /mnt/persistence/wrolpi)" != "$(id -u wrolpi)" ]; then
+  # newer ISO would leave unwritable; re-own an existing library once.
+  if [ -d /mnt/persistence/wrolpi ] \
+      && [ "$(stat -c %u /mnt/persistence/wrolpi)" != "$(id -u wrolpi)" ]; then
     progress "Updating library ownership for this WROLPi version..."
     chown -R wrolpi:wrolpi /mnt/persistence/wrolpi
     note_action "Updated the library's ownership for this WROLPi version"
   fi
+  mkdir -p /mnt/persistence/wrolpi
   chown wrolpi:wrolpi /mnt/persistence/wrolpi
 
   # persistence.conf: live-boot reads this on boot and applies the bind