refactor: best_effort log for quiet steps, registered temp dirs, one exit handler

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ss5gKj7XG5JPi27XwGBGTs

lib/steamos-extras.sh

@@ -40,7 +40,7 @@ extras_enable() {
     done
 
     local tmp_dir src
-    tmp_dir="$(mktemp -d)"
+    make_tmpdir tmp_dir
     if ! fetch_valve_presets "$tmp_dir"; then
         rm -rf "$tmp_dir"
         return 1