Handle upstream iOS simulator slice changes

[leehack]

Summary

• discover iOS xcframework slices from CLiteRTLM.xcframework Info.plist instead of hard-coding the old universal simulator folder
• keep arm64 device and arm64 simulator required, but package x64 simulator only when upstream ships it
• update release validation/docs so v0.13.1 can publish without an unavailable x64 simulator archive

Validation

• python3 tools/package_ios_runtime.py --upstream-tag v0.13.1 --archive --clean
• python3 tools/package_ios_runtime.py --upstream-tag v0.12.0 --clean
• python3 -m py_compile tools/package_ios_runtime.py tools/validate_runtime_artifacts.py tools/validate_release_manifest.py
• git diff --check