Define the versioning and release policy

[Glittersup]

Problem. All three code repos sit at 0.1.0 with nothing stating what a version means, whether the ABI follows semver, how a breaking change reaches the backend and frontend, or what 1.0 requires. SECURITY.md says "pre-1.0" without defining the bar.

Acceptance criteria

• The policy states how the contract ABI is versioned and how breaks are communicated.
• The criteria for 1.0 are written down.