fix(simulation): thread body_params through DeriveStepCanvas terrain derive

The T-1184 merge seam: TerrainAnalysisCache::get_or_derive gained a
body_params parameter (real moisture ceiling for the hydrology solve);
the DeriveStepCanvas arm now passes its own body_params so step-canvas
requests get the same hydrology-carrying TerrainAnalysis the legacy
DeriveWindow path does. Acceptance gate + window goldens green
post-merge.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/gen_queue.rs

@@ -1183,10 +1183,12 @@ fn run_work_item(
                 // on body_id alone, so a step-canvas request and a legacy
                 // district_window request for the same body share one
                 // ~45 ms run_layer1 re-derive, never pay it twice.
+                // body_params threads the real moisture ceiling into the
+                // T-1184 hydrology solve (merge seam resolution).
                 let (l1, ta) = terrain_cache
                     .lock()
                     .unwrap()
-                    .get_or_derive(body_id, &working);
+                    .get_or_derive(body_id, &working, Some(body_params.as_ref()));
                 let climate = ClimateConstants::default();
                 let raw = crate::atlas::step_canvas::build_step_canvas(
                     *body_seed,