feat(editor): thermal erosion simulation for natural terrain aging

- Thermal Erosion: physically-based material transfer where steep
  slopes shed material to neighbors based on angle of repose
- Configurable iterations (1-50) and talus angle (10-80 degrees)
- Lower talus angle = more aggressive erosion (sandy terrain)
- Higher angle = less erosion (rocky terrain holds steep slopes)
- Creates natural talus fans at cliff bases and rounded hilltops
- Workflow: sculpt/generate → thermal erosion → smooth → auto-paint

tools/editor/terrain_editor.hpp

@@ -102,6 +102,9 @@ public:
     // Terrace/quantize heights into N steps
     void terraceHeights(int steps);
 
+    // Thermal erosion: material falls downhill based on angle of repose
+    void thermalErosion(int iterations, float talusAngle);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);