tui: Fix differential rendering to preserve scrollback buffer

- renderDifferential now correctly handles content that exceeds viewport
- When changes are above viewport, do full re-render with scrollback clear
- When changes are in viewport, do partial re-render from change point
- All tests pass, correctly preserves 100 items in scrollback
- Issue: Still re-renders too much (entire tail from first change)

packages/tui/package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@mariozechner/pi-tui",
-	"version": "0.5.7",
+	"version": "0.5.8",
 	"description": "Terminal User Interface library with differential rendering for efficient text-based applications",
 	"type": "module",
 	"main": "dist/index.js",