Fix ZeroDivisionError crash in Particle.update() (SLOP-450)

[tosfos]

Summary

• asteroids_game.py Particle.update() divided by zero once a particle's lifetime countdown reached 0, killing the whole game during normal play shortly after any asteroid explosion (explode() spawns particles with lifetimes of 20-40 frames).
• Guard the fade computation: alpha is set to 0 once the particle expires, mirroring the existing safe pattern in particle.py.

Evidence

Verified empirically with a headless harness driving the actual class from this file (pygame-ce 2.5.8, Python 3.14):

• Before: initial lifetime 25 -> frames 1-24 fine -> frame 25 raises ZeroDivisionError: division by zero.
• After: particles run past expiry with no exception; final alpha is 0 as intended.

Fixes SLOP-450

Test plan

• Harness: particle survives lifetime countdown to/below 0 without raising
• python -m py_compile asteroids_game.py

[coderabbitai]

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