Seasonal: fix summer FPS, richer sun, visible…

Details

- Perf: summer dropped the cursor mesh to ~33fps because the sun rebuilt
  ~15 gradients/frame. Now the sun is composited once to an offscreen
  canvas and blitted, atmosphere gradients are cached, and the ambient
  sim is capped to ~33fps. All seasons measure ~60fps again (verified).
- Sun: replaced the simplistic glow with a limb-darkened disc, soft
  corona/bloom, faint lens-flare halo rings and ~46 irregular baked
  god-rays — reads as an actual sun.
- Winter wind: added flow-field-driven 'gust' streak layers (blowing
  snow) plus stronger, gustier wind, so the wind is visibly felt and
  moves particles naturally along the turbulent current.

Files changed (1)