feat(editor): auto-paint terrain by height bands

- Auto-Paint by Height: automatically sets base texture per chunk based
  on average height — configurable thresholds for sand/grass/rock/snow
- Uses Tanaris sand, Elwynn grass, Barrens rock, Dragonblight snow
- One-click to texture an entire procedurally generated terrain
- Adjustable height thresholds via drag floats
- Workflow: noise → smooth → clamp → auto-paint for instant biome

tools/editor/editor_ui.cpp

@@ -551,6 +551,32 @@ void EditorUI::renderTexturePaintPanel(EditorApp& app) {
             ImGui::TextColored(ImVec4(0.5f, 0.9f, 0.5f, 1.0f), "Active: %s",
                                selectedTexture_.c_str());
 
+        // Auto-paint by height
+        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");
+            static float h1 = 90, h2 = 110, h3 = 140;
+            ImGui::DragFloat("Sand/Dirt max##ap", &h1, 1.0f);
+            ImGui::DragFloat("Grass max##ap", &h2, 1.0f);
+            ImGui::DragFloat("Rock max##ap", &h3, 1.0f);
+            ImGui::Text("Above %.0f: Snow", h3);
+            if (ImGui::Button("Apply Auto-Paint", ImVec2(-1, 0))) {
+                std::vector<TexturePainter::HeightBand> bands = {
+                    {h1, "Tileset\\Tanaris\\TanarisSandBase01.blp"},
+                    {h2, "Tileset\\Elwynn\\ElwynnGrassBase.blp"},
+                    {h3, "Tileset\\Barrens\\BarrensRock01.blp"},
+                    {99999.0f, "Tileset\\Expansion02\\Dragonblight\\DragonblightFreshSmoothSnowA.blp"}
+                };
+                app.getTexturePainter().autoPaintByHeight(bands);
+                // Force terrain refresh
+                auto mesh = app.getTerrainEditor().regenerateMesh();
+                // Mark all chunks dirty through a dummy edit
+                app.showToast("Auto-painted by height");
+            }
+        }
+
+        ImGui::Separator();
+
         // Show textures on chunk under cursor
         auto& brush = app.getTerrainEditor().brush();
         if (brush.isActive()) {

tools/editor/texture_painter.cpp

@@ -165,6 +165,35 @@ std::vector<int> TexturePainter::paint(const glm::vec3& center, float radius,
     return modified;
 }
 
+void TexturePainter::autoPaintByHeight(const std::vector<HeightBand>& bands) {
+    if (!terrain_ || bands.empty()) return;
+
+    // Ensure all band textures are in the texture list
+    for (const auto& band : bands)
+        ensureTextureInList(band.texturePath);
+
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap()) continue;
+
+        // Find average height of this chunk
+        float avgH = chunk.position[2];
+        float sum = 0;
+        for (int v = 0; v < 145; v++) sum += chunk.heightMap.heights[v];
+        avgH += sum / 145.0f;
+
+        // Find which band this chunk falls into
+        for (const auto& band : bands) {
+            if (avgH <= band.maxHeight) {
+                uint32_t texId = ensureTextureInList(band.texturePath);
+                if (!chunk.layers.empty())
+                    chunk.layers[0].textureId = texId;
+                break;
+            }
+        }
+    }
+}
+
 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

@@ -17,6 +17,10 @@ public:
     const std::string& getActiveTexture() const { return activeTexture_; }
     const std::vector<std::string>& getRecentTextures() const { return recentTextures_; }
 
+    // Auto-paint textures based on terrain height bands
+    struct HeightBand { float maxHeight; std::string texturePath; };
+    void autoPaintByHeight(const std::vector<HeightBand>& bands);
+
     // 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);