terrain: add regional USGS 1 m package builder

[vanfleet-dev]

Summary

• add a bounded internal USGS 3DEP 1 m / local-raster preparation CLI
• preserve the existing SRTM converter and shared quantized-mesh encoder
• publish region.json plus truthful fresh terrain/layer.json availability
• derive sparse per-level coverage from Map3D fine-radius/ring LOD behavior
• record source URLs, checksums, CRS, elevation reference, nodata policy, and build metadata

Verification

• 9 passed in scripts/terrain/test_terrain_to_qmesh.py
• real TNM selection rejected newer no-data products and selected four complete 2022 CO DRCOG tiles
• built and decoded 130/130 zoom 13-18 terrain tiles; heights were finite (1584.88-2592.66 m)
• all 180 adjacent same-level edges were within 0.022 m
• the real package loaded through MAVProxy Map3D with all expected local LOD levels

This is a draft while the paired MAVProxy runtime change completes final vehicle/replay edge acceptance.