feat: Implement feature graph and JSON document save/load

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

src/Feature.h

@@ -0,0 +1,25 @@
+#ifndef FEATURE_H
+#define FEATURE_H
+
+#include <QString>
+#include <QJsonObject>
+
+class Feature
+{
+public:
+    Feature(const QString& name);
+    virtual ~Feature() = default;
+
+    QString name() const;
+    void setName(const QString& name);
+
+    virtual QString type() const = 0;
+
+    virtual void read(const QJsonObject &json);
+    virtual void write(QJsonObject &json) const;
+
+private:
+    QString m_name;
+};
+
+#endif // FEATURE_H