feat: Implement feature graph and JSON document save/load

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/SketchFeature.cpp

@@ -0,0 +1,44 @@
+#include "SketchFeature.h"
+
+SketchFeature::SketchFeature(const QString& name)
+    : Feature(name)
+{
+}
+
+QString SketchFeature::type() const
+{
+    return "Sketch";
+}
+
+void SketchFeature::setPlane(SketchPlane plane)
+{
+    m_plane = plane;
+}
+
+SketchFeature::SketchPlane SketchFeature::plane() const
+{
+    return m_plane;
+}
+
+void SketchFeature::read(const QJsonObject& json)
+{
+    Feature::read(json);
+    if (json.contains("plane") && json["plane"].isString()) {
+        QString planeStr = json["plane"].toString();
+        if (planeStr == "XY") m_plane = SketchPlane::XY;
+        else if (planeStr == "XZ") m_plane = SketchPlane::XZ;
+        else if (planeStr == "YZ") m_plane = SketchPlane::YZ;
+    }
+}
+
+void SketchFeature::write(QJsonObject& json) const
+{
+    Feature::write(json);
+    QString planeStr;
+    switch (m_plane) {
+        case SketchPlane::XY: planeStr = "XY"; break;
+        case SketchPlane::XZ: planeStr = "XZ"; break;
+        case SketchPlane::YZ: planeStr = "YZ"; break;
+    }
+    json["plane"] = planeStr;
+}