Version domain descriptors and provider contracts

[malon64]

Context

Domain descriptors and provider contracts are core public interfaces but do not yet have explicit schema versions or migration rules.

Scope

• Add apiVersion and kind to domain descriptors.
• Publish a machine-validated domain schema.
• Add schema_version to provider contracts.
• Remove provider-specific Polaris physical names from domain intent.
• Define compatibility and migration rules.

Acceptance criteria

• Every domain descriptor validates against a versioned schema.
• Every environment exports a versioned provider contract.
• Physical catalog names are derived from provider contracts.
• CI rejects unsupported schema versions.
• Migration instructions are documented.

Roadmap placement

Release: v0.1-alpha.

[malon64]

Completed and merged in #64.