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/test/multi-layout.ts

@@ -3,7 +3,7 @@ import { TUI, Container, TextComponent, TextEditor, MarkdownComponent } from "..
 
 /**
  * Multi-Component Layout Demo
- * 
+ *
  * Demonstrates:
  * - Complex layout with multiple containers
  * - Header, sidebar, main content, and footer areas
@@ -65,13 +65,6 @@ ui.addChild(mainContent);
 ui.addChild(footer);
 ui.setFocus(inputArea);
 
-// Configure debug logging
-ui.configureLogging({
-	enabled: true,
-	level: "info",
-	logFile: "tui-debug.log",
-});
-
 // Handle Ctrl+C to exit
 ui.onGlobalKeyPress = (data: string) => {
 	if (data === "\x03") {