When wind is zero or near-zero, dust particles were always spawning at
the left viewport edge (the "upwind" edge for wind >= 0). Without wind
force to carry them across, they accumulated on the left side. Now
particles spawn across the full viewport when wind is calm (< 5 px/s²),
with random drift directions for even distribution.
Closes#2
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
