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).

apple/scripts/package-core.sh

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+#
+# Packages the prebuilt Apple core into a single zip + checksum, for attaching to
+# the GitHub Release. Lets a consumer (e.g. Xcode Cloud) use the compiled core
+# instead of installing Rust and running build-core.sh. Run AFTER the core exists
+# (apple/scripts/build-core.sh, or `make apple-core` / `make build-apple-dmg`).
+#
+# The bundle carries both build outputs of build-core.sh:
+#   - vnidrop.xcframework   (static libs for device/sim/macOS + the FFI module)
+#   - Vnidrop.swift         (generated UniFFI bindings — a plain source file, not
+#                            part of the xcframework, so it must ship alongside)
+#
+# Produces (under apple/dist):
+#   VnidropCore-<version>.zip
+#   VnidropCore-<version>.zip.sha256   (sha256sum(1)/shasum-compatible format)
+#
+# Zip layout (root):
+#   vnidrop.xcframework/
+#   Vnidrop.swift
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+APPLE_DIR="$REPO_ROOT/apple"
+PKG_DIR="$APPLE_DIR/VnidropCore"
+XCFRAMEWORK="$PKG_DIR/vnidrop.xcframework"
+BINDINGS="$PKG_DIR/Sources/VnidropCore/Vnidrop.swift"
+DIST_DIR="$APPLE_DIR/dist"
+
+VERSION="$("$REPO_ROOT/packaging/version/resolve-version.sh" product)"
+NAME="VnidropCore-$VERSION"
+ZIP="$DIST_DIR/$NAME.zip"
+CHECKSUM="$ZIP.sha256"
+
+[ -d "$XCFRAMEWORK" ] || {
+	echo "error: missing xcframework: $XCFRAMEWORK" >&2
+	echo "       build the core first (apple/scripts/build-core.sh)." >&2
+	exit 1
+}
+[ -f "$BINDINGS" ] || {
+	echo "error: missing generated bindings: $BINDINGS" >&2
+	echo "       build the core first (apple/scripts/build-core.sh)." >&2
+	exit 1
+}
+
+mkdir -p "$DIST_DIR"
+rm -f "$ZIP" "$CHECKSUM"
+
+# Stage a clean tree so the zip root holds exactly the two payloads (no absolute
+# paths or stray parent directories leak into the archive).
+STAGE="$(mktemp -d)"
+trap 'rm -rf "$STAGE"' EXIT
+cp -R "$XCFRAMEWORK" "$STAGE/vnidrop.xcframework"
+cp "$BINDINGS" "$STAGE/Vnidrop.swift"
+
+# -X drops extra file attributes for a stabler archive across machines.
+( cd "$STAGE" && zip -q -r -X "$ZIP" vnidrop.xcframework Vnidrop.swift )
+
+# sha256sum on Linux; shasum -a 256 on macOS. Both emit "<hash>  <name>", which
+# `sha256sum --check` (used by assemble-release.sh) accepts.
+(
+	cd "$DIST_DIR"
+	if command -v sha256sum >/dev/null 2>&1; then
+		sha256sum "$NAME.zip" > "$NAME.zip.sha256"
+	else
+		shasum -a 256 "$NAME.zip" > "$NAME.zip.sha256"
+	fi
+)
+
+echo "==> Packaged prebuilt core"
+echo "    zip:      $ZIP"
+echo "    checksum: $CHECKSUM"