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-options.test.ts

@@ -29,7 +29,7 @@ class EmptyContent implements Component {
 async function renderAndFlush(tui: TUI, terminal: VirtualTerminal): Promise<void> {
 	tui.requestRender(true);
 	await new Promise<void>((resolve) => process.nextTick(resolve));
-	await terminal.flush();
+	await terminal.waitForRender();
 }
 
 describe("TUI overlay options", () => {