Update comparison doc: add XenoAtom.Terminal.UI, refresh control tables

- Add XenoAtom.Terminal.UI as fourth library throughout all comparison tables
- Add dedicated Rendering Architecture section highlighting compositor differences
- Add Gradient Backgrounds section with code example
- Update stale entries: TableControl is now native (was via Spectre wrapper),
  ProgressBarControl is native, add ImageControl/ToolbarControl/NotificationSystem
- Update Layout, Developer Experience, and Project Health sections
- Update "When NOT to Use" and "When SharpConsoleUI Shines" sections

Author: nickprotop