fix(tui): sync test awaits with throttled render pipeline (#3076)

Tests assumed renders complete synchronously after requestRender() +
nextTick + flush(), but commit 6f5f37f8 changed requestRender() to
schedule via setTimeout with a 16ms minimum interval. Tests reading
viewport state before doRender() runs saw stale data.

Add VirtualTerminal.waitForRender() helper that waits for the throttled
render to settle (nextTick + 20ms + xterm flush), and replace all
terminal.flush() calls after render-triggering actions with it.

Fixes 8 failing tests across overlay-short-content, tui-render, and
markdown test suites.

packages/tui/test/overlay-short-content.test.ts

@@ -33,8 +33,7 @@ describe("TUI overlay with short content", () => {
 
 		// Trigger render
 		tui.start();
-		await new Promise((r) => process.nextTick(r));
-		await terminal.flush();
+		await terminal.waitForRender();
 
 		const viewport = terminal.getViewport();
 		const hasOverlay = viewport.some((line) => line.includes("OVERLAY"));