First phase of the D-243 re-scale. Adds server/src/atlas/scale.rs as the
single source of truth for the metric containment ladder (voxel 1m -> chunk
64m -> block 128m -> quarter 512m -> district 2048m -> region 204.8km), the
fixed integer addressing (ChunkPos -> DistrictPos -> RegionPos), and the
elastic seam regions_per_equator(R). Compile-time const asserts enforce the
nesting so the rungs cannot silently drift again — the structural fix for the
Q-110 failure mode (three files, three different region sizes).
Wires body_radius_km (D-204) through BodyParams + the reader: the single
body-specific input to the elastic seam.
No behavior change — foundation only. The rename/re-scale of the carrier
(RegionProfile -> DistrictProfile) and on-demand district derivation follow.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>