S0a: project build — Gradle, engine pin, boundary test, CI #2

Part of #17. Design: docs/design.md §7.

Scope

The build every other slice compiles in.

• Gradle (wrapper checked in) with a Java 21 toolchain. Not raised, no --enable-preview, no gradle.properties overriding the toolchain.
• The engine pin. One relixEngineVersion in the root build.
  • develop tracks 1.0.0-SNAPSHOT from Central's snapshot repository (https://central.sonatype.com/repository/maven-snapshots/), which relix-core publishes on every push to its develop. The CLI's needs then reach the engine before 1.0.0-rc8 is cut (relix-core milestone 1.0.0-rc8). The snapshot repository is added only when the pin is a snapshot.
  • Releases resolve from Maven Central.
  • A checkEnginePin task in check fails a release build that pins a -SNAPSHOT.
• The engine's artifacts (Publish the engine as smaller artifacts: relix, relix-docs, relix-solver-ojalgo, and a relix-all bundle relix-core#84): relix for the command and the library; relix-docs for relix-console's reference-page index; relix-solver-ojalgo at runtime only, so the command's OPTIMIZE and COVER EXACT work.
• Building against a local engine checkout documented as a Gradle composite build (includeBuild).
• A boundary test. Every source set, tests included, may use only the packages the engine jar's module descriptor exports. The modular build gets this from the compiler; the test covers what runs on a class path.
• CI on GitHub Actions for every pull request and every push to main/develop:
  • ./gradlew clean build on Linux, macOS and Windows
  • Dependabot for Gradle and Actions
• Javadoc in check, with dangling references an error.
• Coverage through JaCoCo.
• CONTRIBUTING.md gains the build gate commands.

Dependencies

• Publish the engine as smaller artifacts: relix, relix-docs, relix-solver-ojalgo, and a relix-all bundle relix-core#84 published to the snapshot repository.

Acceptance

• A clean checkout builds green on all three platforms in CI.
• The boundary test fails on a deliberate import of a non-exported engine package; that is probed once when written.
• checkEnginePin fails a release version pinned to a snapshot.