Mark GitHub Release distribution available

[konojunya]

Summary

• mark all four verified native targets and the GitHub Release channel as available
• keep Homebrew, Cargo, Aqua, and self-update explicitly planned
• link the supported v0.3.0 release from the README, distribution guide, and security policy
• update contract tests so partial target activation and premature package-manager activation fail closed

Release evidence

• v0.3.0 resolves to cb1ec1f
• the release contains exactly 19 assets and is neither draft nor prerelease
• all 17 checksummed materials, the keyless checksum signature, four provenance attestations, and four SPDX SBOM attestations verified after public download
• the public macOS arm64 archive passed metadata, signature, version, help, init, check, and render validation

Verification

• 42 Node contract and workflow tests passed
• 10 Python release packaging and Mach-O normalization tests passed
• distribution, supply-chain, and release workflow validators passed
• git diff --check passed