feat(editor): auto-paint by slope for natural cliff texturing

- Auto-Paint by Slope: paints rock texture on steep terrain surfaces
  with configurable slope threshold (0.1 - 0.9)
- Uses per-vertex normal Z component to detect steepness
- Alpha blending based on slope gradient for smooth transitions
- Workflow: sculpt terrain → recalc normals → auto-paint slope → rock
  appears naturally on cliffs while flat areas keep their biome texture

tools/editor/editor_ui.cpp

@@ -756,6 +756,17 @@ void EditorUI::renderTexturePaintPanel(EditorApp& app) {
                                selectedTexture_.c_str());
 
         // Auto-paint by height
+        if (ImGui::CollapsingHeader("Auto-Paint by Slope")) {
+            static float slopeThresh = 0.4f;
+            ImGui::SliderFloat("Slope Threshold", &slopeThresh, 0.1f, 0.9f);
+            ImGui::TextColored(ImVec4(0.6f,0.6f,0.6f,1), "Paints rock on steep slopes");
+            if (ImGui::Button("Apply Slope Paint", ImVec2(-1, 0))) {
+                app.getTexturePainter().autoPaintBySlope(slopeThresh,
+                    "Tileset\\Barrens\\BarrensRock01.blp");
+                app.showToast("Slope paint applied");
+            }
+        }
+
         if (ImGui::CollapsingHeader("Auto-Paint by Height")) {
             ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1),
                 "Sets base texture per chunk based on average height");

tools/editor/texture_painter.cpp

@@ -194,6 +194,45 @@ void TexturePainter::autoPaintByHeight(const std::vector<HeightBand>& bands) {
     }
 }
 
+void TexturePainter::autoPaintBySlope(float slopeThreshold, const std::string& steepTexture) {
+    if (!terrain_) return;
+    uint32_t steepTexId = ensureTextureInList(steepTexture);
+
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap() || chunk.layers.empty()) continue;
+
+        // Compute average slope from normals
+        float maxSlope = 0.0f;
+        for (int v = 0; v < 145; v++) {
+            float nz = static_cast<float>(chunk.normals[v * 3 + 2]) / 127.0f;
+            float slope = 1.0f - std::abs(nz);
+            maxSlope = std::max(maxSlope, slope);
+        }
+
+        if (maxSlope > slopeThreshold) {
+            // Add steep texture as a layer
+            int layerIdx = ensureLayerOnChunk(ci, steepTexId);
+            if (layerIdx > 0) {
+                size_t off = chunk.layers[layerIdx].offsetMCAL;
+                if (off + 4096 <= chunk.alphaMap.size()) {
+                    for (int ty = 0; ty < 64; ty++) {
+                        for (int tx = 0; tx < 64; tx++) {
+                            // Sample slope at this texel
+                            int vi = (ty / 8) * 17 + (tx / 8);
+                            if (vi >= 145) vi = 144;
+                            float nz = static_cast<float>(chunk.normals[vi * 3 + 2]) / 127.0f;
+                            float slope = 1.0f - std::abs(nz);
+                            float alpha = std::clamp((slope - slopeThreshold * 0.5f) / (1.0f - slopeThreshold * 0.5f), 0.0f, 1.0f);
+                            chunk.alphaMap[off + ty * 64 + tx] = static_cast<uint8_t>(alpha * 255.0f);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
 std::vector<int> TexturePainter::erase(const glm::vec3& center, float radius,
                                         float strength, float falloff) {
     if (!terrain_ || activeTexture_.empty()) return {};

tools/editor/texture_painter.hpp

@@ -21,6 +21,9 @@ public:
     struct HeightBand { float maxHeight; std::string texturePath; };
     void autoPaintByHeight(const std::vector<HeightBand>& bands);
 
+    // Auto-paint steep slopes with rock texture
+    void autoPaintBySlope(float slopeThreshold, const std::string& steepTexture);
+
     // Paint the active texture at the given world position
     // Returns list of modified chunk indices
     std::vector<int> paint(const glm::vec3& center, float radius, float strength, float falloff);