feat(editor): reset-to-flat button for terrain, consolidation

- "Reset to Flat" button in Noise Generator section: zeroes all heights
  across entire tile for starting over without creating a new terrain
- Useful workflow: reset → noise → smooth → scale → clamp → auto-paint

tools/editor/editor_ui.cpp

@@ -479,8 +479,12 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
                 app.getTerrainEditor().scaleHeights(hScale);
                 app.showToast("Heights scaled");
             }
+            if (ImGui::Button("Reset to Flat", ImVec2(-1, 0))) {
+                app.getTerrainEditor().resetToFlat();
+                app.showToast("Terrain reset to flat");
+            }
             ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1),
-                "Exaggerate (>1) or flatten (<1) terrain relief");
+                "Scale: exaggerate (>1) or flatten (<1) relief");
         }
 
         ImGui::Separator();

tools/editor/terrain_editor.cpp

@@ -652,6 +652,18 @@ void TerrainEditor::smoothEntireTile(int iterations) {
     dirty_ = true;
 }
 
+void TerrainEditor::resetToFlat() {
+    if (!terrain_) return;
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap()) continue;
+        chunk.heightMap.heights.fill(0.0f);
+        dirtyChunks_.push_back(ci);
+    }
+    for (int ci = 0; ci < 256; ci++) stitchEdges(ci);
+    dirty_ = true;
+}
+
 void TerrainEditor::scaleHeights(float factor) {
     if (!terrain_) return;
     for (int ci = 0; ci < 256; ci++) {

tools/editor/terrain_editor.hpp

@@ -60,6 +60,9 @@ public:
     // Clamp all heights to a min/max range
     void clampHeights(float minH, float maxH);
 
+    // Reset all heights to zero (flat terrain)
+    void resetToFlat();
+
     // Scale all heights by a factor (useful for exaggerating or flattening)
     void scaleHeights(float factor);