fix(simulation): Layer-1 self-describing grid dims + Moore basin trace (#960)

Two Layer-1 generation fixes surfaced by the per-layer atlas viewer:

- Layer1Output now carries grid_w/grid_h (the downsampled working-grid the
  positions live in). The client maps overlays from these, so the scale is
  correct for any source heightmap resolution rather than assuming the texture
  size — fixes overlays projecting at half scale into a corner.
- Drainage basin boundaries are traced as ordered, non-self-crossing contours
  via Moore-neighbour tracing instead of an angle-from-centroid sort. The sort
  produced star-shaped, self-crossing polygons for concave basins that rendered
  as straight chords across the map.

Golden (cascade_layer1.json) and the cross-language atlas_response_ready
fixture regenerated. 100 atlas lib tests + the new tracer test pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/atlas/layer_proxy.rs

@@ -70,6 +70,10 @@ pub fn handle_atlas_request(
             river_network: state.river_network.clone(),
             drainage_basins: state.drainage_basins.clone(),
             attractors: state.attractors.clone(),
+            // The cascade ran on the downsampled heightmap, so its dims are the
+            // working grid all Layer-1 positions are expressed in (#960).
+            grid_w: state.heightmap_width,
+            grid_h: state.heightmap_height,
         };
         return AtlasLayerResponse {
             body_id: req.body_id.clone(),