feat: add twd.waitFor() polling utility - #212

Summary

• Adds twd.waitFor(callback, options?) — a general-purpose retry utility that polls a callback until it stops throwing or the timeout expires (default 2000ms, 50ms interval)
• Replaces blind twd.wait(ms) calls with condition-based waiting: resolves as soon as the condition is met, making tests faster and more reliable
• Supports DOM assertions, analytics event checks, and any custom state check — sync or async callbacks

New API

await twd.waitFor(() => {
  const event = findEvent("purchase"); // your analytics helper
  expect(event).to.exist;
}, { message: "purchase event to fire" });

Options: timeout (ms), interval (ms), message (shown in timeout errors).

Test Plan

• 9 new unit tests in src/tests/utils/waitFor.spec.ts — all passing
• Full suite: 376 tests across 54 files, all passing
• TypeScript compilation clean
• VitePress docs build clean

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