Adds tooling/planet-gen/populate_terrain_reference.py and runs it against
systems.db. Resolves each body's expected wiki heightmap path (repo-root
relative) and writes it into bodies.terrain_reference. Missing heightmaps
are logged for remediation.
Result: 2380/3240 bodies populated, 860 still missing heightmaps. This
unblocks generate_atlas.py (#832) for every body that has a heightmap.
