release: reproducible builds + upgrade compatibility policy

[TheNewAutonomy]

Goal

Establish a release process suitable for mainnet: versioning, reproducible builds, and upgrade compatibility checks.

Scope

• Versioning policy (node + protocol)
• Build reproducibility guidance (CI artifacts)
• Upgrade/rollback story and compatibility gates
• Backwards/forwards compatibility tests for wire formats

Acceptance criteria

• Releases are published with checksums
• A compatibility gate exists in CI for protocol/wire changes