Seasonal iter5: light-theme particle contrast…

Details

Round-3 critique follow-ups (the remaining defects were all light/cream
theme contrast):
- Spring petals + summer motes/seeds: deepen the light-theme palettes
  (rose/magenta + amber) so pale particles read on cream, matching the
  fix already applied to fall leaves.
- Winter wind: raise the streak length floor + factor so gusts read as
  sweeping wind instead of short dashes (both themes).
- Spring: enlarge petals so the cherry-blossom tip/notch resolves
  instead of looking like ovals at small sizes.
Still ~60fps in all seasons.

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