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/file-browser.ts

@@ -0,0 +1,55 @@
+import { TUI, SelectList } from "../src/index.js";
+import { readdirSync, statSync } from "fs";
+import { join } from "path";
+
+const ui = new TUI();
+ui.start();
+let currentPath = process.cwd();
+
+function createFileList(path: string) {
+	const entries = readdirSync(path).map((entry) => {
+		const fullPath = join(path, entry);
+		const isDir = statSync(fullPath).isDirectory();
+		return {
+			value: entry,
+			label: entry,
+			description: isDir ? "directory" : "file",
+		};
+	});
+
+	// Add parent directory option
+	if (path !== "/") {
+		entries.unshift({
+			value: "..",
+			label: "..",
+			description: "parent directory",
+		});
+	}
+
+	return entries;
+}
+
+function showDirectory(path: string) {
+	ui.clear();
+
+	const entries = createFileList(path);
+	const fileList = new SelectList(entries, 10);
+
+	fileList.onSelect = (item) => {
+		if (item.value === "..") {
+			currentPath = join(currentPath, "..");
+			showDirectory(currentPath);
+		} else if (item.description === "directory") {
+			currentPath = join(currentPath, item.value);
+			showDirectory(currentPath);
+		} else {
+			console.log(`Selected file: ${join(currentPath, item.value)}`);
+			ui.stop();
+		}
+	};
+
+	ui.addChild(fileList);
+	ui.setFocus(fileList);
+}
+
+showDirectory(currentPath);