feat(photon): upgrade spectrum-ts sidecar to v7.0.0

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

plugins/platforms/photon/cli.py

@@ -376,11 +376,12 @@ def _install_sidecar() -> int:
         return 1
     # spectrum-ts is pinned exactly in package.json/package-lock.json because
     # the SDK ships breaking majors (v2 removed defineFusorPlatform; v3
-    # reworked space construction). Upgrades are deliberate: bump the pin,
-    # migrate sidecar/index.mjs, re-run the photon tests — never `@latest`
-    # (see README "Upgrading spectrum-ts"). `npm ci` installs the committed
-    # lockfile verbatim; fall back to `npm install` when the lockfile is
-    # missing or drifted (e.g. a dev checkout mid-upgrade).
+    # reworked space construction; v5 split it into @spectrum-ts/* packages).
+    # Upgrades are deliberate: bump the pin, migrate sidecar/index.mjs, re-run
+    # the photon tests — never `@latest` (see README "Upgrading spectrum-ts").
+    # `npm ci` installs the committed lockfile verbatim; fall back to
+    # `npm install` when the lockfile is missing or drifted (e.g. a dev
+    # checkout mid-upgrade).
     print(f"  $ cd {_SIDECAR_DIR} && {npm} ci")
     proc = subprocess.run(  # noqa: S603
         [npm, "ci"],