Fix five quality findings from second re-review

sitecustomize.py: when find_spec resolves real_spec as None (module absent
after a TrueNAS update), record a FAIL status and mark the module done so
hook_status.json is still written and cmd_verify shows a diagnostic FAIL
instead of the ambiguous "no status file found".

sitecustomize.py: the AttributeError fallback in the URL-fix wrapper now
writes a WARNING to stderr before returning the unmodified result, making
the unexpected ResticConfig type visible in journalctl.

apply.sh: after falling back to bare python3, verify that python3 can also
import middlewared; if not, emit a second warning so the operator knows the
backend patch may be installed in the wrong site-packages directory.

patch_ui.py: abort (return without writing) when FIND.subn produces a count
other than 1, instead of committing a doubly-patched bundle and having
subsequent runs silently accept it via the MARKER check.

uninstall.sh: when a vendor sitecustomize.py backup exists, use mv to
atomically overwrite our file rather than rm-then-mv; eliminates the window
where a read-only /usr causes rm to fail under set -e, aborting before the
backup is restored.

uninstall.sh

@@ -62,13 +62,15 @@ SITE_PKG=$("$PYTHON" -c "import site; print(site.getsitepackages()[0])" 2>/dev/n
 
 if [ -n "$SITE_PKG" ] && [ -f "$SITE_PKG/sitecustomize.py" ]; then
     if grep -q "truecloud-patch" "$SITE_PKG/sitecustomize.py" 2>/dev/null; then
-        rm "$SITE_PKG/sitecustomize.py"
-        echo "  Removed $SITE_PKG/sitecustomize.py"
-
         if [ -f "$SITE_PKG/sitecustomize.py.pre-truecloud-patch" ]; then
+            # mv atomically overwrites our file with the vendor original —
+            # safer than rm-then-mv if /usr is transiently read-only.
             mv "$SITE_PKG/sitecustomize.py.pre-truecloud-patch" \
                "$SITE_PKG/sitecustomize.py"
             echo "  Restored previous sitecustomize.py"
+        else
+            rm "$SITE_PKG/sitecustomize.py"
+            echo "  Removed $SITE_PKG/sitecustomize.py"
         fi
     else
         echo "  $SITE_PKG/sitecustomize.py is not ours; leaving it alone."