fix: align macOS install assets and support docs

[HalfAnElephant]

Current problem

macOS support is implemented in code and CI, but the public install/documentation path is inconsistent:

1. install.sh maps macOS to darwin-arm64 / darwin-x86_64 tarball names.
2. .github/workflows/release.yml publishes macOS ARM64 tarballs named macosx-arm64.
3. README and docs still describe macOS LLVM support as planned or Linux-only in places.

This can make the one-line installer fail on macOS and makes the support status unclear.

Expected behavior

• The one-line installer should download the macOS release asset name that the release workflow actually publishes.
• README and getting-started/toolchain docs should describe the current macOS LLVM support accurately.
• A lightweight regression check should cover the macOS asset naming path.

Proposed scope

• Align install.sh macOS platform IDs with release artifacts.
• Keep Linux asset names unchanged.
• Update docs to say macOS ARM64 is supported through LLVM/Clang, while Linux-only packaging modes remain Linux-only.
• Add a focused script/test check for install platform mapping.

Out of scope

• Implementing macOS dylib packaging in mcpp pack.
• Adding Windows support.
• Adding macOS x86_64 release CI unless maintainers want to support that artifact explicitly.

Environment

• Current repository head: b5ce5b1
• Observed host: macOS arm64