feat: Implement feature graph and JSON document save/load

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

src/SketchFeature.h

@@ -0,0 +1,29 @@
+#ifndef SKETCHFEATURE_H
+#define SKETCHFEATURE_H
+
+#include "Feature.h"
+
+class SketchFeature : public Feature
+{
+public:
+    enum class SketchPlane {
+        XY,
+        XZ,
+        YZ
+    };
+
+    SketchFeature(const QString& name);
+
+    QString type() const override;
+
+    void setPlane(SketchPlane plane);
+    SketchPlane plane() const;
+
+    void read(const QJsonObject &json) override;
+    void write(QJsonObject &json) const override;
+
+private:
+    SketchPlane m_plane;
+};
+
+#endif // SKETCHFEATURE_H