Stats and Achievement System

Files
file	achievement_notifier.gd
	Reusable queue for compact achievement unlock notifications.
file	achievement_screen.gd
	Reusable achievement collection popup controller.
file	stats_manager.gd
	Reusable stats, records, and flags manager.
file	stats_panel.gd
	Reusable compact popup renderer for game stats and progression counters.

Detailed Description

Tracks counters, records, flags, derived stats, batched stat events, run summaries, and data-defined achievements.