feat(simulation): load canonical 16-bit grayscale heightmap.png + downsample (D-202 #963)

Replace the atlas_body_heightmaps DB-BLOB loader with a per-body file
loader (D-202 amendment): heightmap.rs reads the 16-bit grayscale
heightmap.png (via the new png dep) from the body's terrain_reference
path, normalizes to f32 [0,1], and rejects RGB so a reliefmap can't be
misread as elevation. Adds BodyHeightmap::downsample (box-average,
deterministic) so Layer 1 drops the high-res stored heightmap to the
512x256 working resolution. sea_level becomes body metadata carried
alongside, not in the PNG. 4 loader tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/Cargo.toml

@@ -29,6 +29,7 @@ sysinfo = "0.35"
 serde_json = "1"
 rusqlite = { version = "0.32", features = ["bundled"] }
 bytemuck = "1"
+png = "0.17"
 toml = "0.8"
 aho-corasick = "1"
 # Economics simulation — Leontief + tâtonnement + D-180 event port (#821)