fix: Resolve compilation errors in parallel and cache modules

- Fix parallel execution logic to properly handle JoinHandle<Result<R, E>> types
- Use join_all instead of try_join_all for proper Result handling
- Fix double question mark (??) issue in parallel execution methods
- Clean up unused imports in parallel and cache modules
- Ensure all performance optimization modules compile successfully
- Fix CI build failures caused by compilation errors

debian/apt-ostreed/DEBIAN/prerm

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -e
+# Automatically added by dh_installsystemd/13.24.2
+if [ -z "$DPKG_ROOT" ] && [ "$1" = remove ] && [ -d /run/systemd/system ] ; then
+	deb-systemd-invoke stop 'apt-ostreed.service' 'apt-ostreed.socket' >/dev/null || true
+fi
+# End automatically added section