feat(editor): road flattener tool alongside river carver

- River/Road Carver section now has two modes:
  - River: carves a channel below terrain (existing)
  - Road: flattens terrain to interpolated height between start/end
- Road mode smoothly transitions height along the path with quadratic
  falloff at edges for natural embankment shape
- Set Start → Set End + Apply workflow works for both modes
- Roads follow terrain slope by interpolating between start/end heights
- Pair with Paint mode to add cobblestone/dirt texture on the road

tools/editor/terrain_editor.cpp

@@ -752,6 +752,50 @@ void TerrainEditor::carveRiver(const glm::vec3& start, const glm::vec3& end,
     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);
+    glm::vec2 lineEnd(end.x, end.y);
+    glm::vec2 lineDir = glm::normalize(lineEnd - lineStart);
+    float lineLen = glm::length(lineEnd - lineStart);
+
+    // Interpolate height along the path
+    auto heightAtT = [&](float t) -> float {
+        return start.z + (end.z - start.z) * (t / lineLen);
+    };
+
+    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);
+            glm::vec2 p(pos.x, pos.y);
+            glm::vec2 toP = p - lineStart;
+            float t = glm::dot(toP, lineDir);
+            t = std::clamp(t, 0.0f, lineLen);
+            glm::vec2 closest = lineStart + lineDir * t;
+            float dist = glm::length(p - closest);
+
+            if (dist < width) {
+                float targetH = heightAtT(t);
+                float relTarget = targetH - chunk.position[2];
+                float falloff = 1.0f - (dist / width);
+                falloff = falloff * falloff;
+                float h = chunk.heightMap.heights[v];
+                chunk.heightMap.heights[v] = h + (relTarget - h) * falloff;
+                modified = true;
+            }
+        }
+        if (modified) {
+            stitchEdges(ci);
+            dirtyChunks_.push_back(ci);
+        }
+    }
+    dirty_ = true;
+}
+
 void TerrainEditor::copyStamp(const glm::vec3& center, float radius) {
     if (!terrain_) return;
     stampData_.clear();