fix: align raster overlays with Web Mercator

[wilhel1812]

Summary

• sample raster overlay rows in Web Mercator space so MapLibre image overlays align with GeoJSON contours
• apply the shared correction to heatmap, target line background, pass/fail, relay, and terrain raster overlays
• add regression coverage for projected midpoint latitude sampling

Closes #902

Verification

• npm test -- --run src/lib/overlayRaster.test.ts
• npm test
• npm run build
• git diff --check
• npm run dev:check