feat(editor): heightmap export, help overlay, keyboard reference

- Export Heightmap: File > Export Heightmap saves terrain as 16-bit
  RAW grayscale (129x129) for use in external terrain editors or
  as a backup. Configurable max height scale.
- Help overlay (F1 or Help menu): lists all keyboard shortcuts
  organized by category (navigation, editing, object transform, view)
- Round-trip heightmap workflow: import → edit → export

tools/editor/terrain_editor.hpp

@@ -54,8 +54,9 @@ public:
     // Noise generator: applies procedural height noise to the terrain
     void applyNoise(float frequency, float amplitude, int octaves, uint32_t seed);
 
-    // Import heightmap from raw 16-bit grayscale (129x129 or 257x257)
+    // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
+    bool exportHeightmap(const std::string& path, float heightScale);
 
     // Water editing
     void setWaterLevel(const glm::vec3& center, float radius, float waterHeight, uint16_t liquidType = 0);