feat(editor): hill/valley generator with smooth bell curve shape

- Hill Generator: creates smooth bell-curve hills at cursor position
  with configurable radius and height
- Valley mode: same shape inverted, creates natural depressions
- Uses (1-t^2)^2 falloff for very smooth natural-looking slopes
- Two buttons: "Create Hill" and "Create Valley" for quick terrain
  shaping without switching brush modes

tools/editor/editor_ui.cpp

@@ -551,6 +551,22 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
                 ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1), "No stamp copied");
         }
 
+        if (ImGui::CollapsingHeader("Hill Generator")) {
+            static float hillRadius = 40.0f, hillHeight = 25.0f;
+            ImGui::SliderFloat("Radius##hill", &hillRadius, 10.0f, 150.0f);
+            ImGui::SliderFloat("Height##hill", &hillHeight, 5.0f, 100.0f);
+            auto& brushH = app.getTerrainEditor().brush();
+            if (ImGui::Button("Create Hill", ImVec2(120, 0)) && brushH.isActive()) {
+                app.getTerrainEditor().createHill(brushH.getPosition(), hillRadius, hillHeight);
+                app.showToast("Hill created");
+            }
+            ImGui::SameLine();
+            if (ImGui::Button("Create Valley", ImVec2(120, 0)) && brushH.isActive()) {
+                app.getTerrainEditor().createHill(brushH.getPosition(), hillRadius, -hillHeight);
+                app.showToast("Valley created");
+            }
+        }
+
         if (ImGui::CollapsingHeader("Mesa / Plateau")) {
             static float mesaRadius = 40.0f, mesaHeight = 20.0f, mesaSteep = 0.3f;
             ImGui::SliderFloat("Radius##mesa", &mesaRadius, 10.0f, 150.0f);

tools/editor/terrain_editor.cpp

@@ -840,6 +840,26 @@ void TerrainEditor::createMesa(const glm::vec3& center, float radius, float heig
     dirty_ = true;
 }
 
+void TerrainEditor::createHill(const glm::vec3& center, float radius, float height) {
+    if (!terrain_) return;
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap()) continue;
+        bool modified = false;
+        for (int v = 0; v < 145; v++) {
+            glm::vec3 pos = chunkVertexWorldPos(ci, v);
+            float dist = glm::length(glm::vec2(pos.x - center.x, pos.y - center.y));
+            if (dist >= radius) continue;
+            float t = dist / radius;
+            float blend = (1.0f - t * t) * (1.0f - t * t); // smooth bell curve
+            chunk.heightMap.heights[v] += height * blend;
+            modified = true;
+        }
+        if (modified) { stitchEdges(ci); dirtyChunks_.push_back(ci); }
+    }
+    dirty_ = true;
+}
+
 void TerrainEditor::flattenRoad(const glm::vec3& start, const glm::vec3& end, float width) {
     if (!terrain_) return;
     glm::vec2 lineStart(start.x, start.y);

tools/editor/terrain_editor.hpp

@@ -87,6 +87,9 @@ public:
     // Create a mesa/plateau (raised flat area with steep cliff edges)
     void createMesa(const glm::vec3& center, float radius, float height, float edgeSteepness);
 
+    // Create a smooth hill/mountain
+    void createHill(const glm::vec3& center, float radius, float height);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);