v0.2: Formik adapter (formdraft/formik)

[mayrang]

Idea

Add a Formik-specific adapter that mirrors the existing formdraft/rhf pattern. Formik is at 3.85M weekly downloads; formik-persist (Jared Palmer's own package) has been unmaintained since 2018. The market gap is verified.

Proposed API

```tsx
import { useFormik } from 'formik';
import { useFormDraftFormik } from 'formdraft/formik';
import { zodAdapter } from 'formdraft';

const formik = useFormik({
initialValues: { name: '', bio: '' },
onSubmit: async (v) => api.submitProfile(v),
});

const { status, lastSavedAt, discard } = useFormDraftFormik(formik, {
key: 'profile-form',
schema: zodAdapter(Schema),
sync: api.saveProfile,
});
```

Implementation notes

• Same pattern as `useFormDraftRHF` (~50 LOC)
• Subscribe to `formik.values` via `useEffect` (no `watch` API in Formik like RHF)
• On restore, call `formik.setValues(draft.values)`
• Guard against the watch→reset feedback loop (see `hasRestoredRef` pattern in RHF adapter)

File structure

• `src/formik/index.ts`
• `src/formik/useFormDraftFormik.ts`
• `src/formik/tests/useFormDraftFormik.test.tsx`
• Add subpath export in `package.json`
• Add tsup entry

Tests

Mirror the 2 RHF adapter tests:

1. Persists Formik values on input change
2. Restores into Formik on mount when storage has valid draft

Acceptance

• New subpath export `formdraft/formik`
• 2+ unit tests passing
• README mentions Formik integration in install section
• Bundle still under 8 KB (Formik adapter goes into its own chunk)