Document Homebrew distribution availability

[konojunya]

Summary

• mark the owner-maintained Homebrew channel available for the supported macOS and Linux targets
• document install, upgrade, uninstall, and recovery ownership
• keep immutable v0.3.0 release metadata distinct from the later Homebrew activation
• extend contract validation so only GitHub Releases and Homebrew are currently active

Testing

• node scripts/validate-distribution-contract.mjs
• node --test scripts/distribution-contract.test.mjs
• cargo fmt --check
• cargo test --locked
• git diff --check