feat(waitFor): make generic to return callback value - #214

Summary

• Make twd.waitFor() generic (<T>) so it returns the callback's return value instead of void
• Fully backwards compatible — existing void callbacks still work identically
• Update API docs and AI guides with new signature, examples, and "only use when retry logic is necessary" warning

Usage

// Before: all assertions must go inside the callback
await twd.waitFor(() => {
  const event = findEvent("purchase");
  expect(event).to.exist;
  expect(event.customer_type).to.equal("b2c");
});

// After: extract the value and assert outside
const event = await twd.waitFor(() => {
  const ev = findEvent("purchase");
  expect(ev).to.exist;
  return ev;
});
expect(event.customer_type).to.equal("b2c");

Test plan

• 3 new unit tests (sync return, async return, return after retries)
• All 379 existing tests pass
• TypeScript compiles clean in example app (tsc --noEmit)
• Build succeeds
• VitePress docs build succeeds

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