Update README for interact() and version 4.1.

[djscruggs]

What

• Bump the recommended install version in the README from 3.x to 4.1 (unpkg <script> tag and package.json snippet).
• Move the deprecated get()/store() walkthrough (get(), store(), WebCredential, Handling Empty Results) out of the README into docs/get-store.md; the README keeps a short deprecation callout that links to it for existing integrations.
• Link "interaction URL" to the VCALM specification (Interaction URL Format) in the interact() section and the deprecation callout.
• Update the "Advanced Operations for Wallet Providers" intro: relying parties use interact() now, and wallet providers do not call navigator.credentials at all — registration uses CredentialManager.requestPermission() (which goes through navigator.credentialsPolyfill.permissions), and handlers receive requests via events or a protocols query parameter. This means a future removal of the navigator.credentials patching is a no-op for wallets.

Notes

• Docs-only change; no source or test changes.
• The "Features" section and the interact() under-the-hood paragraph still reference navigator.credentials.get()/store() — accurate for the current implementation; they should be revisited when the navigator.credentials patching is actually removed.
• CI runs lint (Node 22.x) and the Playwright test matrix; both npm run lint and npm test pass locally (20 passed, 1 skipped WebKit case).

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