fix(tui): fix scrollback overwrite when appending lines past viewport

Appended lines were not committed to terminal scrollback because the
renderer used cursor movement (CSI B) and carriage return without
linefeed. This caused earlier content to be overwritten when the
viewport filled up.

Changes:
- For appended lines, emit \r\n to create real scrollback lines
- When target row is below viewport, scroll with \r\n before positioning
- Add PI_TUI_WRITE_LOG env var for debugging raw ANSI output
- Add fullRedraws readonly property to TUI class
- Add viewport-overwrite-repro.ts test script

fixes #954

packages/coding-agent/docs/tui.md

@@ -443,6 +443,14 @@ interface MyTheme {
 }
 ```
 
+## Debug logging
+
+Set `PI_TUI_WRITE_LOG` to capture the raw ANSI stream written to stdout.
+
+```bash
+PI_TUI_WRITE_LOG=/tmp/tui-ansi.log npx tsx packages/tui/test/chat-simple.ts
+```
+
 ## Performance
 
 Cache rendered output when possible: