Replace 3K+ React CircleMarker components + MarkerClusterGroup with
native Leaflet L.circleMarker on a shared L.canvas() renderer added
in a single useEffect. Zero React components per marker — canvas
draws all points in one paint (<50ms vs multi-second freeze).
Also drops react-leaflet-cluster from the bundle (-46KB gzipped).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>