docs(simulation): fix stale elevation comment in derive_all_regions

Re-review (Hoshe + Tyre, both APPROVE) caught one stale comment: after the
per-region elevation fix, derive_region_profile owns elevation_km (from elev_q),
not the caller. Update the comment so maintainers don't read it as caller-supplied.

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

server/src/atlas/region_profile.rs

@@ -654,9 +654,10 @@ pub fn derive_all_regions(
         };
         for rx in 0..region_cols {
             let pos = (rx, ry);
-            // Build per-region params: body-level params + per-region lat/elev.
-            // elevation_km for the region is approximated from body_params.elevation_km
-            // (caller-level); per-cell refinement happens at ChunkContext (D-239).
+            // Build per-region params: body-level params + this region's latitude.
+            // Per-region elevation is NOT set here — derive_region_profile owns it,
+            // deriving elevation_km from the region's own elev_q (not the caller's
+            // body_params.elevation_km). Per-cell refinement happens at ChunkContext (D-239).
             let region_params = BodyParams {
                 region_latitude_deg: lat_deg,
                 ..body_params.clone()