feat(simulation): district moisture spatial gradient (T-1080)

moisture_q was a single body constant (derive_moisture_q took only &BodyParams →
80 for every one of 2048 ocean-world districts), so vegetation/terrain/ecotones
were uniform — "nothing to fuzz" (the T-1080 believability bug).

- district_profile.rs: derive_moisture_q is now a body ceiling (hydrosphere+atmosphere)
  × per-district gradient — latitude (equator wet → pole dry), elevation (orographic/
  rain-shadow), continentality (100 − ocean_fraction_q). Integer/D-010, no new plumbing
  (elev_q/ocean_fraction_q/latitude_deg already reach build_district_profile). Mirrors
  D-240's temperature model. Coefficients in ClimateConstants + climate_constants.toml
  [moisture_gradient] (provisional; Q-123 calibrates). Tuned so a wet body stays mostly
  green with drier patches and a frozen body clamps mostly barren. + gradient unit test.
- region_profile.rs: region-tier moisture gets the region-latitude gradient (elevation/
  continentality are district-scale).
- Propagates to precipitation_class / vegetation_class / morphology_zone (all read
  moisture_q). D-239 §2 amended. Believability golden regenerated.

Probe (Arbour @ yolo): moisture_q distinct 1 → 49 (spread 0 → 48); 5/7 → 6/7 criteria.
Edict (frozen): moisture 0..20 ×21 — gradient within its low ceiling. Remaining fail
(vegetation-present) is the metric dividing by all-sampled incl. water on a 2/3-ocean
world — a Q-123 calibration concern, not a climate defect.

clippy --all-targets -D warnings clean; 1580 lib tests + harnesses pass.

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

server/tests/golden/believability.json

@@ -6,9 +6,9 @@
     "voxel_sampled_districts": 64,
     "contrast": {
       "moisture_q": {
-        "min": 80,
+        "min": 32,
         "max": 80,
-        "distinct": 1
+        "distinct": 49
       },
       "elev_q": {
         "min": 0,
@@ -27,13 +27,13 @@
       },
       "morphology_zones": 9,
       "vegetation_classes": 3,
-      "terrain_materials": 4
+      "terrain_materials": 3
     },
     "coherence": {
       "water_districts": 46,
-      "water_districts_wet": 43,
+      "water_districts_wet": 44,
       "drainage_samples": 10,
-      "drainage_monotonic": 8,
+      "drainage_monotonic": 9,
       "vegetation_samples": 64,
       "vegetated_districts": 11
     }
@@ -45,9 +45,9 @@
     "voxel_sampled_districts": 64,
     "contrast": {
       "moisture_q": {
-        "min": 20,
+        "min": 0,
         "max": 20,
-        "distinct": 1
+        "distinct": 21
       },
       "elev_q": {
         "min": 0,