Seasonal iter3: light-theme visibility, denser…

Details

Address critique-agent feedback:
- Winter: deep slate-blue light palette so snow/wind read on the cream
  theme; more & longer, always-present gust streaks (blowing snow).
- Summer: move the sun fully on-screen (was clipped at the top edge);
  boost corona/bloom/ray alpha on the light theme so it doesn't wash out.
- Fall: more leaves, higher flurry floor, better spread.
- Spring: stronger/larger pollen, deeper green (visible on cream), and a
  proper rounded petal shape with a crease instead of a flat ellipse.
All seasons still measure ~60fps with the cursor mesh active.

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