Add Apple SPM XCFramework release assets

[leehack]

Summary

• Package Apple LiteRT-LM runtime payloads from litert-lm-native@v0.13.1 as SwiftPM-compatible XCFramework zip assets.
• Build the macOS runtime with an explicit 14.0 deployment target so downstream Swift packages can set a matching platform floor.
• Fix macOS x64 runtime builds on Apple Silicon by passing explicit Bazel x86_64 CPU/platform settings; without this, the x64 build path could produce arm64 output.
• Package LiteRtLm as a framework XCFramework and macOS companion libraries as no-header dylib XCFrameworks to avoid module-map collisions and framework install-name padding issues.
• Preserve symlinks in release zips and document the native-version update checklist for downstream SwiftPM checksum updates.

Validation

• python3 -m py_compile tools/build_upstream_runtime.py tools/package_apple_xcframeworks.py
• Fresh local v0.13.1 macOS builds:
  • python3 tools/build_upstream_runtime.py --upstream-tag v0.13.1 --platform macos --arch arm64 --jobs 8
  • python3 tools/build_upstream_runtime.py --upstream-tag v0.13.1 --platform macos --arch x64 --jobs 8
• Verified fresh Mach-O output:
  • bin/macos/arm64/libLiteRtLm.dylib: arm64, macOS minOS 14.0
  • bin/macos/x64/libLiteRtLm.dylib: x86_64, macOS minOS 14.0
  • bin/macos/x64/libLiteRt.dylib: x86_64, macOS minOS 14.0
• python3 tools/package_apple_xcframeworks.py --upstream-tag v0.13.1 --clean
• git diff --check

Release Notes

• Existing v0.13.1 release assets do not yet include these SwiftPM zips. After merge, publish the native release assets and compute downstream SwiftPM checksums from the exact uploaded zips.