feat(simulation): detail-scatter synthesis + on-demand 2km derive_district (T-1077)

Phase 3 of the D-243 re-scale — the corrected 2km carrier and its 'stretch
magic'.

- detail_scatter.rs: the mid-scale terrain (2-40km) invented between coarse
  heightmap samples. Locked character (T-1077): ADAPTIVE (ruggedness follows
  local slope/morphology -> gentle flats, rugged highs) + ENVELOPE+MODULATION
  (heightmap relief caps amplitude; morphology flavors within). Deterministic
  seed+position value-noise (D-227/D-010); C1-continuous, no lattice creases.
- derive_district(): the on-demand 2km carrier. Maps the district to a
  fractional heightmap pixel via body_radius_km (the elastic seam, D-204),
  bilinearly interpolates the L1 envelope, composes the scatter, derives the
  profile. body_radius None falls back to direct indexing (tiny test bodies).
- Factored build_district_profile() (climate+morphology tail) shared by the
  cell-aggregate and interpolation paths.

5 scatter tests + 5 derive_district tests (determinism, latitude->climate via
the seam, the envelope rule, radius fallback). Full suite + clippy -D warnings
green; existing goldens untouched (additive).

Scope: T-1077 delivers the corrected on-demand 2km derivation + scatter. The
eager-grid -> on-demand region/district production + Atlas surfacing (decision
A) is T-1046's chartered job (derive_all_districts stays the coarse Atlas grid
for now, documented).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/mod.rs

@@ -10,6 +10,7 @@ pub mod body_world_state;
 pub mod cascade;
 pub mod chunk_context;
 pub mod city_context_reader;
+pub mod detail_scatter;
 pub mod district_mix;
 pub mod district_profile;
 pub mod domain_warp;