Extend release-plan validation with version progression checks

[hdamker]

Problem description

The release-plan.yaml validation (PR #48) covers schema validation and basic semantic checks. However, it cannot validate version progression rules that require comparing against previous releases.

Possible evolution

Add the following validation checks:

1. Release Tag Progression

   • Within release cycle: r4.1 → r4.2 → r4.3
   • New cycle: r4.3 → r5.1 (not r5.2)
   • Implementation: Use GitHub API to list existing releases

2. API Semantic Version Progression

   • Draft → Alpha: version can change freely
   • Alpha → RC: minor version bump expected
   • RC → Public: no version change expected
   • Public → Public: proper increment based on changes

3. Maintenance Release Constraints

   • Only patch version bumps allowed (X.Y.Z → X.Y.Z+1)
   • No new APIs can be added
   • All APIs must remain 'public' status

4. Unchanged API Version Lock

   • Version must match the previous public release
   • Status must be 'public'

Implementation approach

Use GitHub API to:

1. List existing releases in the repository
2. Fetch release-metadata.json or release-metadata.yaml from release assets for previous release details (including API versions)

The release-metadata files are uploaded by the release collector for legacy releases and follow the format defined in the release-metadata schema.

Alternative solution

None identified - GitHub API access is required for cross-release validation.

Additional context

• Related PR: Add release-plan validation support #48 (Initial release-plan validation)
• Related issue: Validation: Complete release-plan.yaml validation infrastructure #49 (Release plan validation tracker)
• Priority: Medium - catches configuration errors early but requires GitHub API access