feat(editor): detail noise for adding small-scale terrain roughness

- Detail Noise: adds high-frequency roughness to existing terrain
  without destroying the overall shape (amplitude 0.5-10, freq 0.01-0.5)
- Useful after smooth/generate to break up unnaturally smooth surfaces
- Workflow: generate → smooth → detail noise for natural look
- Separate seed from main noise generator for independent control

tools/editor/terrain_editor.hpp

@@ -123,6 +123,9 @@ public:
     // Ramp tile edges to a target height for seamless multi-tile joins
     void rampEdges(float targetHeight, float rampWidth);
 
+    // Add random detail noise to existing terrain (preserves shape, adds roughness)
+    void addDetailNoise(float amplitude, float frequency, 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);