[codex] Add module-info.java via multi-release JAR

[cowwoc]

What changed

Adds a JPMS module-info.java for com.knuddels.jtokkit while preserving Java 8 compatibility by publishing the library JAR as a multi-release JAR.

Why

The library targets Java 8, so a plain top-level module descriptor would break that baseline. Packaging module-info.class under META-INF/versions/9/ keeps Java 8 consumers unchanged while exposing a named module on Java 9+.

Impact

• Java 8 consumers continue to use the library unchanged.
• Java 9+ consumers can require com.knuddels.jtokkit as a named module.
• The sources JAR now includes module-info.java.

Validation

• JAVA_HOME=/opt/sdkman/candidates/java/21.0.8-zulu PATH=/opt/sdkman/candidates/java/21.0.8-zulu/bin:$PATH java -classpath gradle/wrapper/gradle-wrapper.jar org.gradle.wrapper.GradleWrapperMain :lib:check :lib:jar
• Verified the built JAR contains META-INF/versions/9/module-info.class
• Verified the manifest contains Multi-Release: true