#257: Add Load Game screen to main menu with sorted save list, loading
overlay during quickload round-trip, and pending_load_path cross-scene
flow. F5/F6 quicksave/quickload were already wired.
#561: Move debug_overlay.gd from scripts/ui/ to ui/ for consistency
with all other UI components. Update scene and test references.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
