[perNyfelt]

Adds opt-in root support for releasing several exact, unreleased publications in one Central bundle. Production compile dependencies participate in eligibility, exclusions cascade, BOM pins are checked, and guarded manifests drive allow-listed archives and one-attempt deployment recovery. Existing single-module bundle/release behavior remains available.

The Matrix integration uses an explicit sibling composite build until this enhancement is published separately. The new task contract is documented in combined-releases.md.

Validation:

• ./gradlew build --no-configuration-cache --max-workers=2 passed, including existing separate-release tests.
• Strict configuration-cache TestKit reuse passed for discovery, source guards, nonempty Maven preparation and bundling.
• Unit checks cover Portal/repository status, dependency/BOM exclusions, archive policy, late publication, concurrent local submission, unknown upload outcomes, deployment failure and resume/propagation.
• Matrix acceptance checks exercised real signed Maven installation/flattening, GPG signature verification, immutable staging, BOM API tests and japicmp with isolated fixture repositories.

Live sanitized Portal fixture capture still needs publisher credentials; production absence-based discovery and uploads fail closed until capture confirms the contract. Plugin publication is a separate release step. No artifacts were published and no Git tags were created.

Matrix integration: Alipsa/matrix#476.

Plugin CI passed: https://github.com/Alipsa/nexus-release-plugin/actions/runs/37967189167 (build and Gradle 8.13/9.4 compatibility jobs). Matrix's final cross-platform verification passed: https://github.com/Alipsa/matrix/actions/runs/37969265857.