Update the Photon platform plugin's Node.js sidecar from spectrum-ts
3.1.0 to 7.0.0, which splits the SDK into scoped `@spectrum-ts/*`
packages with `spectrum-ts` as the umbrella re-export.
- Bump exact pin in package.json/package-lock.json to 7.0.0
- Update mixed-attachments patch script to target the new
`@spectrum-ts/imessage/dist/index.js` path and tab-indented output
- Rewrite test fixture to match v7.x mapper shape (tab-indented,
`const ... = async` declarations, single-line builder calls) and
point at `@spectrum-ts/imessage/dist/index.js`
- Update README upgrade guide to document the v5 package split and
the postinstall patch validation step
- Update comments in cli.py and index.mjs to reference v5/v7 changes