feat(editor): gradient texture blend for biome transitions

- Gradient Blend: smoothly transitions base texture from one biome
  to another across the entire tile (horizontal or vertical)
- Preset buttons: Grass→Sand and Grass→Snow gradients
- Creates natural biome transition zones without manual painting
- Alpha blending from 0% to 100% across 16 chunks

tools/editor/editor_ui.cpp

@@ -897,6 +897,26 @@ void EditorUI::renderTexturePaintPanel(EditorApp& app) {
                                selectedTexture_.c_str());
 
         // Auto-paint by height
+        if (ImGui::CollapsingHeader("Gradient Blend")) {
+            static int gradDir = 0;
+            ImGui::RadioButton("Horizontal##grad", &gradDir, 0);
+            ImGui::SameLine();
+            ImGui::RadioButton("Vertical##grad", &gradDir, 1);
+            if (ImGui::Button("Grass → Sand Gradient", ImVec2(-1, 0))) {
+                app.getTexturePainter().gradientBlend(
+                    "Tileset\\Elwynn\\ElwynnGrassBase.blp",
+                    "Tileset\\Tanaris\\TanarisSandBase01.blp", gradDir == 0);
+                app.showToast("Gradient applied");
+            }
+            if (ImGui::Button("Grass → Snow Gradient", ImVec2(-1, 0))) {
+                app.getTexturePainter().gradientBlend(
+                    "Tileset\\Elwynn\\ElwynnGrassBase.blp",
+                    "Tileset\\Expansion02\\Dragonblight\\DragonblightFreshSmoothSnowA.blp", gradDir == 0);
+                app.showToast("Gradient applied");
+            }
+            ImGui::TextColored(ImVec4(0.6f,0.6f,0.6f,1), "Smooth texture transition across tile");
+        }
+
         if (ImGui::CollapsingHeader("Auto-Paint by Slope")) {
             static float slopeThresh = 0.4f;
             ImGui::SliderFloat("Slope Threshold", &slopeThresh, 0.1f, 0.9f);

tools/editor/texture_painter.cpp

@@ -289,6 +289,38 @@ void TexturePainter::paintAlongPath(const glm::vec3& start, const glm::vec3& end
     }
 }
 
+void TexturePainter::gradientBlend(const std::string& tex1, const std::string& tex2, bool horizontal) {
+    if (!terrain_) return;
+    uint32_t id1 = ensureTextureInList(tex1);
+    uint32_t id2 = ensureTextureInList(tex2);
+
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap() || chunk.layers.empty()) continue;
+        int cx = ci % 16, cy = ci / 16;
+
+        // Set base texture to tex1
+        chunk.layers[0].textureId = id1;
+
+        // Add tex2 as blended layer
+        int layerIdx = ensureLayerOnChunk(ci, id2);
+        if (layerIdx < 0) continue;
+        size_t off = chunk.layers[layerIdx].offsetMCAL;
+        if (off + 4096 > chunk.alphaMap.size()) continue;
+
+        for (int ty = 0; ty < 64; ty++) {
+            for (int tx = 0; tx < 64; tx++) {
+                float t;
+                if (horizontal)
+                    t = (cx * 64.0f + tx) / (16.0f * 64.0f);
+                else
+                    t = (cy * 64.0f + ty) / (16.0f * 64.0f);
+                chunk.alphaMap[off + ty * 64 + tx] = static_cast<uint8_t>(t * 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

@@ -28,6 +28,9 @@ public:
     void paintAlongPath(const glm::vec3& start, const glm::vec3& end,
                         float width, const std::string& texturePath);
 
+    // Gradient blend: transitions base texture from one to another across tile
+    void gradientBlend(const std::string& tex1, const std::string& tex2, bool horizontal);
+
     // 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);