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/tests/gen_fixtures.rs

@@ -561,6 +561,8 @@ fn generate_atlas_layer_response_fixtures() {
             sub_biome: SubBiomeVariant::CoastalLowland,
             terrain_modification_cost: 1.7,
         }],
+        grid_w: 512,
+        grid_h: 256,
     };
     let ready = AtlasLayerResponse {
         body_id: "GJ1c".into(),