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/layer1.rs

@@ -32,6 +32,12 @@ pub struct Layer1Output {
     /// Geographic attractors (D-209) with sub-biome + cost (D-210), sorted by
     /// `(attractor_type, row, col)`.
     pub attractors: Vec<GeographicAttractor>,
+    /// Working-grid dimensions every position in this output (river cells, basin
+    /// boundaries, attractor positions) is expressed in — equals the downsampled
+    /// heightmap size. The client maps these onto the displayed heightmap (#960),
+    /// so the overlay scale stays correct for any source resolution (mod-safe).
+    pub grid_w: u32,
+    pub grid_h: u32,
 }
 
 /// Run the Layer-1 topography pipeline for a single body.
@@ -60,6 +66,8 @@ pub fn run_layer1(hm: &BodyHeightmap) -> Layer1Output {
         river_network: drainage.river_network,
         drainage_basins: drainage.drainage_basins,
         attractors,
+        grid_w: hm.width,
+        grid_h: hm.height,
     }
 }