Refactor version-gate into service modules

[k-barseghyan]

Summary

• split the project into SPI, core, server, testkit, and placeholder PostgreSQL/S3 adapter modules
• move version allocation into the atomic ControlStore.beginBuild contract and remove client-supplied target versions
• add reusable storage contract suites, preserve lifecycle/API behavior, and update architecture, configuration, CI, and contributor documentation
• package the server as an executable Spring Boot JAR with the placeholder adapter modules on its runtime classpath

Validation

• parsed all Java sources and Maven POMs locally
• verified package/path alignment, project imports, module boundaries, and clean diffs locally
• full Java 21 ./mvnw clean verify runs in GitHub Actions

Closes #1