Winter snow: real GPU (WebGL) particle snowfield…

Details

Replace the 2D streamline 'wind' (which read as tadpoles and was slow due
to full-screen destination-out + drawImage every frame) with an actual
GPU particle system:
- New snow-gl.ts: a WebGL point/quad particle snowfield. Thousands of
  soft, depth-layered flakes (near = large soft bokeh, far = small) are
  advected by a curl-noise wind field (procedural divergence-free flow)
  with gusts, so the snow drifts and swirls on the wind like igloo.inc.
- Rendered as quads (two triangles) rather than gl.POINTS, because
  gl_PointSize is clamped to ~1px on SwiftShader/Safari/many GPUs.
- Runs on the GPU, freeing the main thread (fixes the slowdown); caps at
  ~36fps; soft radial-falloff fragment shader; theme-aware color.
- Winter only; falls back to a light 2D snow layer if WebGL is missing.
  Honors the weather toggle (via the weathertoggle event + localStorage),
  reduced-motion, tab visibility, resize/DPR.
Also: 2D winter simplified to a light fallback snow + a few 6-arm
snowflake accents; the old trail/gust streamline system removed.

Files changed (4)