feat(editor): terrain mirror X/Y for symmetric zone design

- Mirror X: copies left half of terrain to right half (mirrored)
- Mirror Y: copies top half to bottom half (mirrored)
- Useful for creating symmetric zones, arenas, or balanced landscapes
- Auto-stitches all chunk edges after mirror for seamless results
- UI buttons in Sculpt panel under "Mirror Terrain" section

tools/editor/editor_ui.cpp

@@ -459,6 +459,17 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
         }
 
         ImGui::Separator();
+        if (ImGui::CollapsingHeader("Mirror Terrain")) {
+            if (ImGui::Button("Mirror X (Left<>Right)", ImVec2(-1, 0))) {
+                app.getTerrainEditor().mirrorX();
+                app.showToast("Terrain mirrored X");
+            }
+            if (ImGui::Button("Mirror Y (Top<>Bottom)", ImVec2(-1, 0))) {
+                app.getTerrainEditor().mirrorY();
+                app.showToast("Terrain mirrored Y");
+            }
+        }
+
         if (ImGui::CollapsingHeader("Stamp / Clone")) {
             auto& brush2 = app.getTerrainEditor().brush();
             if (ImGui::Button("Copy Stamp", ImVec2(120, 0)) && brush2.isActive())

tools/editor/terrain_editor.cpp

@@ -666,6 +666,52 @@ void TerrainEditor::scaleHeights(float factor) {
     dirty_ = true;
 }
 
+void TerrainEditor::mirrorX() {
+    if (!terrain_) return;
+    for (int cy = 0; cy < 16; cy++) {
+        for (int cx = 0; cx < 8; cx++) {
+            int srcIdx = cy * 16 + cx;
+            int dstIdx = cy * 16 + (15 - cx);
+            auto& src = terrain_->chunks[srcIdx];
+            auto& dst = terrain_->chunks[dstIdx];
+            if (!src.hasHeightMap() || !dst.hasHeightMap()) continue;
+            for (int v = 0; v < 145; v++) {
+                int row = v / 17, col = v % 17;
+                if (col > 8) continue;
+                int mirrorCol = 8 - col;
+                int mirrorV = row * 17 + mirrorCol;
+                dst.heightMap.heights[mirrorV] = src.heightMap.heights[v];
+            }
+            dirtyChunks_.push_back(dstIdx);
+        }
+    }
+    for (int ci = 0; ci < 256; ci++) stitchEdges(ci);
+    dirty_ = true;
+}
+
+void TerrainEditor::mirrorY() {
+    if (!terrain_) return;
+    for (int cy = 0; cy < 8; cy++) {
+        for (int cx = 0; cx < 16; cx++) {
+            int srcIdx = cy * 16 + cx;
+            int dstIdx = (15 - cy) * 16 + cx;
+            auto& src = terrain_->chunks[srcIdx];
+            auto& dst = terrain_->chunks[dstIdx];
+            if (!src.hasHeightMap() || !dst.hasHeightMap()) continue;
+            for (int v = 0; v < 145; v++) {
+                int row = v / 17, col = v % 17;
+                if (col > 8) continue;
+                int mirrorRow = 8 - row;
+                int mirrorV = mirrorRow * 17 + col;
+                dst.heightMap.heights[mirrorV] = src.heightMap.heights[v];
+            }
+            dirtyChunks_.push_back(dstIdx);
+        }
+    }
+    for (int ci = 0; ci < 256; ci++) stitchEdges(ci);
+    dirty_ = true;
+}
+
 void TerrainEditor::copyStamp(const glm::vec3& center, float radius) {
     if (!terrain_) return;
     stampData_.clear();

tools/editor/terrain_editor.hpp

@@ -68,6 +68,10 @@ public:
     void pasteStamp(const glm::vec3& center);
     bool hasStamp() const { return !stampData_.empty(); }
 
+    // Mirror terrain along X or Y axis through tile center
+    void mirrorX();
+    void mirrorY();
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);