MULTI-SCALE HILLSHADE UPDATE

The topography renderer now builds hillshade at three raster-pixel scales:

- sigma 0.0, weight 0.55: original DEM; preserves fine ridges, gullies, and tributaries
- sigma 2.0, weight 0.30: medium terrain structure
- sigma 6.0, weight 0.15: broad ridges, valleys, and mountain forms

Each scale uses the existing four-direction multidirectional hillshade. The
three normalized hillshades are combined and then blended into the continuous
elevation colors with the existing soft-light blend at strength 0.20.

This is a display-only operation. It does not alter the source DEM, statistics,
legend elevation values, or downloaded raster.

To compare with old output, delete the relevant backend/cache/<watershed>
folder or use the application's force-refresh path before loading topography.
