Add minimum Gradle compatibility functional test

Upgrade Gradle to 9.4.1 and fix functional tests

Keep contribution-based assemblies on managed Sync tasks, register direct producers through ArtifactAssemblyRegistry, wire builtBy on outgoing artifacts, and cover the behavior with functional tests.

Set the project version to 0.1.0, add publication descriptions/license metadata, and keep module-level docs as compatibility pointers to the root documentation.

variants: move source set layout conventions out of model Keep GenericSourceSet convention-free and apply layout defaults from SourcesPlugin and VariantSourcesPlugin. Add compile unit layout convention and cover standalone and variant source set layouts in functional tests.

variants: expose compile unit source set specs Route variantSources selectors through CompileUnitSourceSetSpec so source set materialization keeps compile unit identity available. Add configureEach hooks and keep GenericSourceSet as a plain source set model.

variants: validate source context identities

variants: refine public API boundary

variants: stabilize artifact slot materialization

variants: enforce source configuration policies