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/editor_ui.cpp

@@ -1470,13 +1470,21 @@ void EditorUI::renderWaterPanel(EditorApp& app) {
             app.setWaterType(static_cast<uint16_t>(typeIdx));
 
         ImGui::Separator();
+        if (ImGui::Button("Fill Entire Tile with Water", ImVec2(-1, 0))) {
+            app.getTerrainEditor().fillWater(app.getWaterHeight(), app.getWaterType());
+            app.showToast("Tile filled with water");
+        }
         if (ImGui::Button("Remove Water Under Brush", ImVec2(-1, 0))) {
             auto& brush = app.getTerrainEditor().brush();
             if (brush.isActive()) {
                 app.getTerrainEditor().removeWater(brush.getPosition(), s.radius);
-                app.getEditorCamera(); // trigger dirty
             }
         }
+        if (ImGui::Button("Remove ALL Water", ImVec2(-1, 0))) {
+            for (int ci = 0; ci < 256; ci++)
+                app.getTerrainEditor().getTerrain()->waterData[ci].layers.clear();
+            app.showToast("All water removed");
+        }
 
         ImGui::Separator();
         ImGui::TextColored(ImVec4(0.5f, 0.8f, 1.0f, 1.0f), "Left-click to place water");