Add type selector popup, view root switching, and new type creation

- Type selector chevron [▸] on command row opens searchable popup
- Popup lists all root structs with filter, keyboard nav, side-triangle indicator
- Selecting a type switches the editor view via setViewRootId
- "Create new type" inserts a new root struct with no name
- Command row displays the active view root's name
- Tests for chevron detection, span compatibility, view switching, undo

src/typeselectorpopup.h

@@ -0,0 +1,58 @@
+#pragma once
+#include <QFrame>
+#include <QFont>
+#include <QVector>
+#include <QString>
+#include <cstdint>
+
+class QLineEdit;
+class QListView;
+class QStringListModel;
+class QLabel;
+class QToolButton;
+
+namespace rcx {
+
+struct TypeEntry {
+    uint64_t id            = 0;
+    QString  displayName;
+    QString  classKeyword;   // "struct", "class", or "enum"
+};
+
+class TypeSelectorPopup : public QFrame {
+    Q_OBJECT
+public:
+    explicit TypeSelectorPopup(QWidget* parent = nullptr);
+
+    void setFont(const QFont& font);
+    void setTypes(const QVector<TypeEntry>& types, uint64_t currentId);
+    void popup(const QPoint& globalPos);
+
+signals:
+    void typeSelected(uint64_t structId);
+    void createNewTypeRequested();
+    void dismissed();
+
+protected:
+    bool eventFilter(QObject* obj, QEvent* event) override;
+    void hideEvent(QHideEvent* event) override;
+
+private:
+    QLabel*           m_titleLabel   = nullptr;
+    QLabel*           m_escLabel     = nullptr;
+    QToolButton*      m_createBtn    = nullptr;
+    QLineEdit*        m_filterEdit   = nullptr;
+    QListView*        m_listView     = nullptr;
+    QStringListModel* m_model        = nullptr;
+
+    QVector<TypeEntry> m_allTypes;
+    QVector<TypeEntry> m_filteredTypes;
+    uint64_t           m_currentId = 0;
+    QFont              m_font;
+
+    void applyFilter(const QString& text);
+    void acceptCurrent();
+    void acceptIndex(int row);
+};
+
+} // namespace rcx