Rework variant artifacts materialization model Refactor VariantArtifactsPlugin around a live outgoing artifacts context and split artifact publication into explicit internal services: outgoing variant registry, assembly binding, materialization policy hooks, primary-slot convention, and slot assembly handling. Introduce variant artifact slots as identity-first public API and expose materialized assembly handles through ArtifactAssemblies. Add replayable configuration hooks for outgoing configurations, outgoing slots, outgoing variants, and registered assemblies. Create consumable outgoing configurations per variant, bind the primary slot to the root outgoing artifact set, and publish non-primary slots as Gradle outgoing configuration variants. Add deterministic injective task names for slot assembly tasks, use Sync for directory assembly, and configure the default assembly output location under build/variant-assemblies. Make primary-slot selection finalize-on-read and provide a single-slot convention that fails when no unique default can be inferred. Mark artifact internal implementation package as non-public API.