fix: align macOS install asset support

[HalfAnElephant]

Summary

• Align install.sh macOS ARM64 platform mapping with the published macosx-arm64 release asset.
• Stop advertising macOS x86_64 in the installer until release CI publishes that asset.
• Update README and getting-started/toolchain docs to describe current macOS ARM64 LLVM support.
• Add an e2e regression check for installer platform mapping.

Closes #50

Test plan

• bash -n install.sh
• bash -n tests/e2e/45_install_platform_mapping.sh
• bash tests/e2e/45_install_platform_mapping.sh
• git diff --check HEAD~1 HEAD

[Sunrisepeak]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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