feat(cache): add onRevalidate callback for SWR completion notification

[marcstraube]

Summary

Closes #8

• Add optional onRevalidate callback to CacheGetOptions<T>, invoked with fresh data when background stale-while-revalidate fetch completes
• Enables reactive UIs to update when revalidation finishes without polling

Usage

const result = await cache.get('api:users', {
  staleWhileRevalidate: true,
  revalidate: () => fetch('/api/users').then((r) => r.json()),
  onRevalidate: (freshData) => {
    renderUsers(freshData);
    hideRefreshIndicator();
  },
});

The callback is not called when:

• Entry is not stale (no revalidation needed)
• Revalidation throws (stale data remains)
• Cache is destroyed before completion
• Revalidation is deduplicated (only first caller's callback fires)

Test plan

• 5 new tests covering callback invocation, error case, destroy, non-stale, deduplication
• All 4228 tests pass, 100% line coverage
• TypeScript strict, ESLint 0 warnings
• Documentation updated in documentation/cache.md

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