feat(editor): sand dune generator for desert terrain

- Dune Generator: creates rolling sand dune patterns with primary
  and secondary sine waves plus hash-based variation
- Configurable wavelength, amplitude, wind direction, and seed
- Directional waves create realistic parallel dune ridges
- Secondary wave adds natural irregularity at 2.3x wavelength
- Perpendicular variation breaks up uniform dune lines
- Pair with Desert biome paint for instant Tanaris-style zones

tools/editor/terrain_editor.hpp

@@ -129,6 +129,9 @@ public:
     // Rotate terrain 90 degrees clockwise
     void rotateTerrain90();
 
+    // Create rolling sand dune pattern
+    void createDunes(float wavelength, float amplitude, float direction, uint32_t seed);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);