feat: Add Habitat plan for macOS aarch64-darwin (Apple Silicon)

[sanghinitin]

Summary

Adds a Habitat package plan for macOS aarch64-darwin (Apple Silicon), enabling hab pkg build on ARM-based Macs.

Changes Made

• Created habitat/aarch64-darwin/plan.sh — full Habitat plan for building ohai on macOS Apple Silicon
• Uses core/clang instead of core/gcc for native gem compilation (gcc unavailable on aarch64-darwin)
• Exports CC/CXX pointing to clang for native extension builds
• Uses DYLD_LIBRARY_PATH (macOS) instead of LD_LIBRARY_PATH (Linux) in the runtime wrapper
• Sets HOME and GEM_SPEC_CACHE to writable Habitat cache paths to avoid touching host user directories in Studio
• Explicitly installs racc gem into the vendor directory — required at runtime by tomlrb (uses require "racc/parser") but not declared as a gem dependency; Ruby 3.4 bundles racc in its own gem path which is hidden by the wrapper GEM_PATH
• Mirrors the existing Linux plan structure: appbundler binstubs, binstub_patch.rb injection, libexec wrapper pattern

Evidence / Validation

• bash -n habitat/aarch64-darwin/plan.sh — syntax check passes
• hab pkg build . — completed successfully on aarch64-darwin
• hab pkg install results/*.hart — installed without errors
• hab pkg exec sanghinitin/ohai ohai -v — returns correct version
• hab pkg exec sanghinitin/ohai ohai — full system detection runs without LoadError
• Verified racc/parser.rb LoadError resolved by explicit racc install in vendor

Root Cause of racc Issue

tomlrb internally does require "racc/parser" but does not declare racc as a gem dependency. In Ruby 3.4, racc is a bundled gem living in Ruby's own gem path. The ohai runtime wrapper sets GEM_PATH exclusively to the vendor directory, hiding Ruby's bundled gems. Explicitly installing racc into the vendor dir resolves this.

Related Issue

CHEF-34741

CHEF-34742

AI Assistance

This work was completed with AI assistance following Progress AI policies.

[neha-p6]

https://github.com/chef/ohai/actions/runs/29721775870/job/88286124866