Fix snapper create-config race: retry the whole dance, not just umount

Confirmed on the test laptop's real install: /etc/snapper/configs/ was
completely empty post-install — snapper create-config failed silently and
BOS's advertised snapshot/rollback feature was entirely non-functional,
despite snapper-cleanup.timer being enabled and grub-btrfsd active (both
harmless no-ops with no config to act on).

Root cause is the known chroot-specific busy-mount race already documented
in this section's comments, but the existing recovery (retry umount 5x,
then one lazy-unmount fallback) wasn't sufficient on this hardware — a
lazy unmount detaches the mountpoint from the namespace immediately, but
whatever was holding it busy can take a moment longer to actually release,
and the immediately-following rmdir/create-config both fail if anything
still references /.snapshots at that instant.

Wrap the entire unmount → rmdir → create-config → cleanup → remount
sequence in an outer retry loop (checking whether the config file actually
exists before each attempt and after the loop), add a settle delay after
the lazy-unmount fallback, and turn the final failure into a loud ERROR
instead of a warning that's easy to miss — a system silently shipping
without snapshots is worse than one that's slow to set them up.

iso/airootfs/etc/calamares/post-install.sh

@@ -272,11 +272,20 @@ fi
 # transiently fail inside the Calamares chroot (the same mount unmounts
 # cleanly moments later once booted normally — a chroot-specific busy-mount
 # race, not a logic error), which cascades into snapper create-config
-# refusing because .snapshots "already exists". Retry a few times, then
-# fall back to a lazy unmount (detaches the mountpoint immediately even if
-# something still transiently references it) rather than give up.
+# refusing because .snapshots "already exists". A single pass — retry the
+# umount a few times, then fall back to a lazy unmount — was NOT enough on
+# real hardware: also confirmed is a run where every umount attempt in that
+# single pass failed (including the lazy fallback settling too slowly for
+# the immediately-following rmdir/create-config), leaving
+# /etc/snapper/configs/ completely empty and BOS's advertised snapshot/
+# rollback feature silently non-functional on that install. So retry the
+# WHOLE dance, not just the umount substep, and verify at the end that the
+# config file actually exists before declaring success.
 # ---------------------------------------------------------------------------
 if command -v snapper &>/dev/null; then
+    for attempt in 1 2 3; do
+        [[ -f /etc/snapper/configs/root ]] && break
+
         unmounted=0
         for _ in 1 2 3 4 5; do
             if umount /.snapshots 2>/dev/null; then
@@ -286,16 +295,25 @@ if command -v snapper &>/dev/null; then
             sleep 1
         done
         if [[ "$unmounted" != "1" ]]; then
-        echo "WARN: umount /.snapshots failed after retries, forcing lazy unmount"
-        umount -l /.snapshots || echo "WARN: lazy umount /.snapshots also failed"
+            echo "WARN: umount /.snapshots failed after retries (attempt $attempt), forcing lazy unmount"
+            umount -l /.snapshots || echo "WARN: lazy umount /.snapshots also failed (attempt $attempt)"
+            # Lazy unmount detaches the mountpoint from the namespace right
+            # away, but whatever was holding it busy may take a moment
+            # longer to actually let go — rmdir/create-config right after
+            # this both fail if anything still references /.snapshots.
+            sleep 2
         fi
-    rmdir /.snapshots || echo "WARN: rmdir /.snapshots failed"
-    snapper -c root create-config / || echo "WARN: snapper create-config failed"
+        rmdir /.snapshots 2>/dev/null || echo "WARN: rmdir /.snapshots failed (attempt $attempt)"
+        snapper -c root create-config / || echo "WARN: snapper create-config failed (attempt $attempt)"
         if [[ -d /.snapshots ]]; then
-        btrfs subvolume delete /.snapshots || echo "WARN: deleting snapper's own .snapshots subvolume failed"
+            btrfs subvolume delete /.snapshots || echo "WARN: deleting snapper's own .snapshots subvolume failed (attempt $attempt)"
         fi
         mkdir -p /.snapshots
-    mount /.snapshots || echo "WARN: remounting the real @snapshots subvolume failed"
+        mount /.snapshots || echo "WARN: remounting the real @snapshots subvolume failed (attempt $attempt)"
+
+        [[ -f /etc/snapper/configs/root ]] || sleep 2
+    done
+
     if [[ -f /etc/snapper/configs/root ]]; then
         sed -i 's/TIMELINE_CREATE="yes"/TIMELINE_CREATE="no"/' /etc/snapper/configs/root
         sed -i 's/NUMBER_CLEANUP="no"/NUMBER_CLEANUP="yes"/' /etc/snapper/configs/root
@@ -304,6 +322,8 @@ if command -v snapper &>/dev/null; then
         sed -i 's/NUMBER_LIMIT_IMPORTANT="[^"]*"/NUMBER_LIMIT_IMPORTANT="5"/' /etc/snapper/configs/root
         [[ -n "$MAIN_USER" ]] && \
             sed -i "s/ALLOW_USERS=\"\"/ALLOW_USERS=\"$MAIN_USER\"/" /etc/snapper/configs/root
+    else
+        echo "ERROR: snapper config for root still missing after 3 attempts — snapshots/rollback will not work on this install"
     fi
 fi