fix(editor): NPC markers always on top, mesa generator, terrain tools

- NPC markers now render with NO depth test (via gizmo pipeline) so
  they're always visible even on sloped/rough terrain
- Mesa/Plateau generator: creates raised flat areas with steep cliff
  edges — configurable radius, height, and edge steepness
- NPC markers drawn after gizmo in the render pipeline to guarantee
  they appear on top of everything
- Fixes NPC visibility on non-flat terrain

tools/editor/terrain_editor.hpp

@@ -84,6 +84,9 @@ public:
     // Create a crater at a position (bowl shape with raised rim)
     void createCrater(const glm::vec3& center, float radius, float depth, float rimHeight);
 
+    // Create a mesa/plateau (raised flat area with steep cliff edges)
+    void createMesa(const glm::vec3& center, float radius, float height, float edgeSteepness);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);