fix(galaga): fix collision detection with batched updates

[burinc]

Critical bug fix: Enemies weren't being destroyed when hit by bullets. This was caused by two interrelated issues in collision detection.

Problems Fixed:

1. STALE DATA: bullets and enemies were captured at the start of update-game!, but collision checks happened AFTER bullet position updates. Result: checking collisions with old positions.

2. RACE CONDITIONS: Each collision in doseq triggered separate swap! calls, but all iterations used the ORIGINAL collections. Multiple collisions on same bullet/enemy caused state corruption.

Solution: Collision Batching

• Capture FRESH bullets/enemies right before collision detection
• Collect all collisions in sets (preventing duplicate processing)
• Apply ALL effects in a single atomic swap! at the end
• Use :when guards to skip already-processed objects

Benefits:

• Enemies now properly destroyed when shot
• Boss enemies correctly track 2-hit damage
• No race conditions or state corruption
• Single re-render per collision batch
• Predictable, deterministic behavior

This uses the same collision batching pattern from Asteroids, inspired by Erik Assum's feedback on atomic state updates.

Also updated article documentation to explain the bug and solution with detailed code examples and credit to Erik Assum.