Change Set 8: The Big Cleanup

A broad sweep across readability, feel, and presentation.

• All garbage now glows one safe green. No more guessing whether something is a threat or a pickup.
• A stronger Magnet and far more powerups: every UFO drops one, plus a bonus at the dock.
• Removed the leftover dashed debug boundary.
• Game Over is now a spectacle: a shockwave that sets off nearby debris, screen shake, and a white flash, instead of everything just freezing.
• Added top-10 local high scores with three-initial entry.
• Cleaned up the HUD layout, and gave large Hunters a slow last-stand pursuit so a wave can’t stall out with one drifting core left.