feat(editor): invert heights, fill entire tile with water, remove all water

- Invert Heights: flips terrain upside-down around midpoint (mountains
  become valleys and vice versa)
- Fill Entire Tile with Water: one-click fills all 256 chunks at the
  configured water height and liquid type
- Remove ALL Water: clears water from every chunk instantly
- Water panel now has three water operations: fill tile, remove under
  brush, remove all
- fillWater() and invertHeights() methods on TerrainEditor

tools/editor/terrain_editor.hpp

@@ -105,6 +105,12 @@ public:
     // Thermal erosion: material falls downhill based on angle of repose
     void thermalErosion(int iterations, float talusAngle);
 
+    // Invert terrain (flip heights around midpoint)
+    void invertHeights();
+
+    // Fill entire tile with water at a height
+    void fillWater(float height, uint16_t liquidType);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);