fix: restore Homebrew formula validation and updater safety

[josedab]

Summary

• Modernizes the formula, makes checksum updates atomic, and adds hermetic updater and structural audit coverage.
• Preserves repository public contracts while restoring or strengthening the verification baseline.
• Includes an AUDIT.md where the repository reached the audit/refactor phase.

Commits

• fac1c10 fix(homebrew): restore formula validation baseline (BREW-D-1)
• e3a8bca docs(homebrew): record clean-code audit

Validation

ruby -c, Bash syntax, ShellCheck, updater success/missing-artifact fixtures, brew audit --strict, and brew style pass.

Known blockers / deferred work

• None beyond required PR checks and documented AUDIT.md deferrals.

Merge readiness

Ready for review. Merge only after all required GitHub checks and reviewer approvals pass.