feat(tui): overlay positioning API with CSS-like values

Add OverlayOptions for configurable positioning (anchor, margins, offsets,
percentages). Add OverlayHandle for programmatic visibility control with
hide/setHidden/isHidden. Add visible callback for responsive overlays.

Extension API: ctx.ui.custom() now accepts overlayOptions and onHandle callback.

Examples: overlay-qa-tests.ts (10 test commands), doom-overlay (DOOM at 35 FPS).

packages/coding-agent/examples/extensions/doom-overlay/README.md

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+# DOOM Overlay Demo
+
+Play DOOM as an overlay in pi. Demonstrates that the overlay system can handle real-time game rendering at 35 FPS.
+
+## Usage
+
+```bash
+pi --extension ./examples/extensions/doom-overlay
+```
+
+Then run:
+```
+/doom-overlay
+```
+
+The shareware WAD file (~4MB) is auto-downloaded on first run.
+
+## Controls
+
+| Action | Keys |
+|--------|------|
+| Move | WASD or Arrow Keys |
+| Run | Shift + WASD |
+| Fire | F or Ctrl |
+| Use/Open | Space |
+| Weapons | 1-7 |
+| Map | Tab |
+| Menu | Escape |
+| Pause/Quit | Q |
+
+## How It Works
+
+DOOM runs as WebAssembly compiled from [doomgeneric](https://github.com/ozkl/doomgeneric). Each frame is rendered using half-block characters (▀) with 24-bit color, where the top pixel is the foreground color and the bottom pixel is the background color.
+
+The overlay uses:
+- `width: "90%"` - 90% of terminal width
+- `maxHeight: "80%"` - Maximum 80% of terminal height
+- `anchor: "center"` - Centered in terminal
+
+Height is calculated from width to maintain DOOM's 3.2:1 aspect ratio (accounting for half-block rendering).
+
+## Credits
+
+- [id Software](https://github.com/id-Software/DOOM) for the original DOOM
+- [doomgeneric](https://github.com/ozkl/doomgeneric) for the portable DOOM implementation
+- [pi-doom](https://github.com/badlogic/pi-doom) for the original pi integration