build(apple): publish prebuilt core bundle in release assets

Bundle the compiled Apple core — vnidrop.xcframework plus the generated UniFFI
bindings (Vnidrop.swift, a source file that lives outside the xcframework) — into
VnidropCore-<version>.zip with a checksum, and attach it to the GitHub Release.
This lets a consumer (e.g. Xcode Cloud, later) use the prebuilt core instead of
installing Rust and running build-core.sh.

No duplicate builds: the release job compiles the core once (build-apple-dmg ->
build-core.sh release), links it into the signed DMG, and package-core.sh only
zips that same output. Package.swift is unchanged (still binaryTarget(path:)).

- apple/scripts/package-core.sh: stage xcframework + Vnidrop.swift and zip them
  with a sha256sum/shasum-compatible checksum sidecar (macOS-native).
- Makefile: package-apple-core target.
- apple-release.yml: run package-apple-core after the DMG and upload the zip +
  checksum in the macOS artifact.
- assemble-release.sh: verify the core zip's checksum, copy it into the final
  assets, and list it in release-manifest.json + SHA256SUMS (+ fixture update).

.github/workflows/apple-release.yml

@@ -134,6 +134,12 @@ jobs:
       - name: Build, sign & notarize DMG
         run: make build-apple-dmg
 
+      - name: Package prebuilt core
+        # build-apple-dmg builds the release Rust core + Swift bindings; bundle them
+        # (xcframework + Vnidrop.swift + checksum) as a release asset so consumers can
+        # skip building the core. See apple/scripts/package-core.sh.
+        run: make package-apple-core
+
       - name: Upload notarization diagnostics
         if: failure()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
@@ -156,5 +162,7 @@ jobs:
             apple/dist/VniDrop-*.dmg
             apple/dist/VniDrop-*.build-info.json
             apple/dist/appcast.xml
+            apple/dist/VnidropCore-*.zip
+            apple/dist/VnidropCore-*.zip.sha256
           if-no-files-found: error
           retention-days: 14