feat(client): add save/load game flow and move debug_overlay (#257, #561)

#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>

client/scripts/autoloads/game_state.gd

@@ -75,6 +75,11 @@ var rng_seed: Variant = null
 # One-shot: consumed by main.gd after display, then set back to null.
 var save_result: Variant = null
 
+# #257: Pending load path — set by main menu "Load Game" selection.
+# main.gd sends LOAD_GAME on startup if non-empty, then clears this field.
+# Format: user://saves/<game-id>/<filename>.sav or "" if no pending load.
+var pending_load_path: String = ""
+
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]