Adds overlay rendering capability to the TUI, enabling floating modal
components that render on top of existing content without clearing the screen.
- Add showOverlay(), hideOverlay(), hasOverlay() methods to TUI
- Implement ANSI-aware line compositing via extractSegments()
- Support overlay stack (multiple overlays, later on top)
- Add { overlay: true } option to ctx.ui.custom()
- Add overlay-test.ts example extension
Also fixes pre-existing bug where bash tool output cached visual lines
at fixed terminal width, causing crashes on terminal resize.
Co-authored-by: Nico Bailon <nico.bailon@gmail.com>
