feat(tui): add overlay compositing for ctx.ui.custom() (#558)

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>

packages/tui/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Overlay compositing for `ctx.ui.custom()` with `{ overlay: true }` option ([#558](https://github.com/badlogic/pi-mono/pull/558) by [@nicobailon](https://github.com/nicobailon))
+
 ## [0.38.0] - 2026-01-08
 
 ### Added