chore(readme): add npm download badges

[mayeedwin]

Summary

• Add three shields.io npm monthly-downloads badges (core, react, angular) directly under the H1, each linking to its npm package page.

Why

• Give visitors an at-a-glance signal of adoption per package. Since this is a monorepo with three published packages, per-package badges are more accurate than a single headline.

Test plan

• Confirm badges render on GitHub in both light and dark themes
• Click each badge → lands on the correct npm package page

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.