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

tsconfig.json

@@ -1,6 +1,7 @@
 {
 	"extends": "./tsconfig.base.json",
 	"compilerOptions": {
+		"noEmit": true,
 		"paths": {
 			"@mariozechner/pi-tui": ["./packages/tui/src/index.ts"],
 			"@mariozechner/pi-agent": ["./packages/agent/src/index.ts"],