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)

package-lock.json

@@ -792,7 +792,7 @@
 		},
 		"packages/agent": {
 			"name": "@mariozechner/pi-agent",
-			"version": "0.5.7",
+			"version": "0.5.8",
 			"license": "MIT",
 			"dependencies": {
 				"@mariozechner/pi-tui": "^0.5.7",
@@ -1236,7 +1236,7 @@
 		},
 		"packages/pods": {
 			"name": "@mariozechner/pi",
-			"version": "0.5.7",
+			"version": "0.5.8",
 			"license": "MIT",
 			"dependencies": {
 				"@mariozechner/pi-agent": "^0.5.7",
@@ -1252,7 +1252,7 @@
 		},
 		"packages/tui": {
 			"name": "@mariozechner/pi-tui",
-			"version": "0.5.7",
+			"version": "0.5.8",
 			"license": "MIT",
 			"dependencies": {
 				"@types/mime-types": "^2.1.4",