Instead of hiding the sentinel tab (which leaked space on macOS),
repurpose it as a visible "+" button that creates a new struct tab
on click. Compact 32px icon-only tab with pixel-perfect cross drawn
via fillRect. Skips context menu and middle-click. Always positioned
as the last tab in the group.
- 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