Add a publish-gradle job to the release workflow to publish Processing libraries to the Gradle Plugin Portal using gradle publish with required secrets and version/group env vars. Update gradle/plugins/library/build.gradle.kts to use the com.gradle.plugin-publish plugin, provide plugin metadata (website, vcsUrl, displayName, description, tags) and make the plugin id dynamic ("$group.library"). These changes enable automated publishing of the Gradle plugin with the metadata required by the portal.

Expose the project's group to test JVMs by configuring tasks.withType<Test>() to set systemProperty("project.group", group ?: "org.processing"). Update ProcessingPluginTest to read the plugin id from System.getProperty("project.group") instead of hardcoding the group. Also close the publishing block in build.gradle.kts. This allows tests to adapt when the project group is overridden.