The canvas overlay hard-coded a 0.0625° half-step when projecting each
cell to canvas pixels — correct for HRRR (0.125° native grid) but wrong
for HRDPS, which we downsample to 0.5° (0.25° half-step). At HRRR's
half-step every HRDPS cell got painted at 1/16th its actual area, so
the overlay over Canada looked like a sparse pinstripe of vertical
streaks instead of contiguous coverage.
Plumb halfStep through the WeatherCanvasOverlay constructor and pick
0.25 vs 0.0625 in the hook based on data-source.
Graham McIntire
fe7d998f67