feat(wind): Level 2 — canvas particle simulation (uniform field)

- ParticleWindView: transparent View overlay on the map; tracks 300 particles
  as three FloatArrays (lat, lon, age) — no per-frame allocation; projects
  lat/lon → screen via MapLibreMap.projection each frame; draws a 14px tail
  segment in the wind travel direction with alpha fading by age; respawns
  out-of-bounds/aged particles randomly within visible bounds; speed scaled
  to viewport so animation is consistent at any zoom level; animates via
  postInvalidateOnAnimation()
- activity_main.xml: ParticleWindView added above MapView, match_parent,
  non-interactive (clickable/focusable=false)
- MainActivity: finds particle_wind_view; calls attachMap after getMapAsync;
  feeds directionDeg/speedKt from windArrow StateFlow

https://claude.ai/code/session_01HXPjBsogsJVRwCiekDGkJX

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