Place the real-elevation heightmap here as:

    puget_heightmap.r16

Generate it with tools/dem_to_heightmap.py from a USGS/SRTM DEM GeoTIFF of the
Tacoma–Seattle–Mt. Rainier region. See docs/TERRAIN_PIPELINE.md.

If this file is absent (or the size doesn't match WorldBuilder.HeightmapRes), the game
falls back to procedural Puget-Sound-style terrain automatically.

The .r16 format: little-endian unsigned 16-bit, row-major, (res x res) where res is
2^n + 1, south row first.
