Spatial wind particles: grid interpolation, fast projection, density/opacity

- Particles now use bilinear interpolation of the 4×5 wind grid so
  speed and direction vary by position, swirling around weather systems.
  Particles are hidden until the first grid arrives (no stale fallback).

- Replace 150–300 per-particle toScreenLocation() JNI calls with a
  single 3-point affine transform computed once per frame. Rendering
  stays smooth during map panning.

- Per-particle tail direction is now derived from the local (u,v) wind
  vector rotated into screen space by the map bearing.

- Density: N 300→600, activeN range [150,600] (was [60,300]).
- Opacity: max alpha 200→110, min 15→8; stroke 3.5→2.5 px.

Wire setWindGrid() in MainActivity windGrid.collect alongside the
existing updateWindGridLayer() call.

https://claude.ai/code/session_017TwkSjhzhmTcHKmVG5MWUa

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