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/markdown.test.ts

@@ -592,7 +592,7 @@ describe("Markdown component", () => {
 			const component = new MarkdownWithInput(markdown);
 			tui.addChild(component);
 			tui.start();
-			await terminal.flush();
+			await terminal.waitForRender();
 
 			assert.ok(component.markdownLineCount > 0);
 			const inputRow = component.markdownLineCount;
@@ -1032,7 +1032,7 @@ bar`,
 			const tui = new TUI(terminal);
 			tui.addChild(markdown);
 			tui.start();
-			await terminal.flush();
+			await terminal.waitForRender();
 
 			const renderedLine = markdown.render(80)[0];
 			assert.ok(renderedLine, "Should render heading line");