fix(tui): Container change detection for proper differential rendering

Fixed rendering artifact where duplicate bottom borders appeared when components
dynamically shifted positions (e.g., Ctrl+C in agent clearing status container).

Root cause: Container wasn't reporting as "changed" when cleared (0 children),
causing differential renderer to skip re-rendering that area.

Solution: Container now tracks previousChildCount and reports changed when
child count changes, ensuring proper re-rendering when containers are cleared.

- Added comprehensive test reproducing the layout shift artifact
- Fixed Container to track and report child count changes
- All tests pass including new layout shift artifact test

packages/tui/src/index.ts

@@ -19,8 +19,6 @@ export { TextComponent } from "./components/text-component.js";
 export { TextEditor, type TextEditorConfig } from "./components/text-editor.js";
 // Whitespace component
 export { WhitespaceComponent } from "./components/whitespace-component.js";
-// Logger for debugging
-export { type LoggerConfig, logger } from "./logger.js";
 // Terminal interface and implementations
 export { ProcessTerminal, type Terminal } from "./terminal.js";
 export {