feat(editor): height offset tool for shifting entire terrain up/down

- Offset Heights: shifts all terrain heights by a constant amount
  (-100 to +100 range with Apply button)
- Useful for raising terrain above water level or sinking below
- Slider + Apply button in the Noise Generator section

tools/editor/editor_ui.cpp

@@ -546,6 +546,13 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
                 app.getTerrainEditor().invertHeights();
                 app.showToast("Heights inverted");
             }
+            static float offsetAmt = 10.0f;
+            ImGui::SliderFloat("Offset##ht", &offsetAmt, -100.0f, 100.0f, "%.0f");
+            ImGui::SameLine();
+            if (ImGui::SmallButton("Apply##off")) {
+                app.getTerrainEditor().offsetHeights(offsetAmt);
+                app.showToast("Heights offset");
+            }
             ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1),
                 "Scale: exaggerate (>1) or flatten (<1) relief");
         }

tools/editor/terrain_editor.cpp

@@ -860,6 +860,18 @@ void TerrainEditor::createHill(const glm::vec3& center, float radius, float heig
     dirty_ = true;
 }
 
+void TerrainEditor::offsetHeights(float amount) {
+    if (!terrain_) return;
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap()) continue;
+        for (int v = 0; v < 145; v++)
+            chunk.heightMap.heights[v] += amount;
+        dirtyChunks_.push_back(ci);
+    }
+    dirty_ = true;
+}
+
 void TerrainEditor::invertHeights() {
     if (!terrain_) return;
     // Find midpoint

tools/editor/terrain_editor.hpp

@@ -108,6 +108,9 @@ public:
     // Invert terrain (flip heights around midpoint)
     void invertHeights();
 
+    // Offset all heights by a constant
+    void offsetHeights(float amount);
+
     // Fill entire tile with water at a height
     void fillWater(float height, uint16_t liquidType);