Validate marker labels against their scopes

[Pybsama]

Summary

• validate SMPTE marker labels against the permitted values for their computed scope
• warn when a marker scope is unknown and restore the 2013 schema default scope
• cover invalid, unknown, and valid 2013/2016/2020 marker labels

Testing

• JDK 11: ./gradlew build
• JDK 17 and JDK 21: ./gradlew --rerun-tasks test --tests com.netflix.imflibrary.validation.IMFCPLValidatorTest
• git diff --check

Closes #394

[Pybsama]

This is now ready for review.

A follow-up edge-case review found that a schema-valid but unsupported TrackFileResourceType inside a MarkerSequence could leave an empty non-marker virtual track. The marker-label validation is now limited to actual IMFMarkerVirtualTrack instances, so the existing empty-resource error is reported instead of a ClassCastException. A regression test covers that path.

Fresh verification on the published tree:

• JDK 11: ./gradlew build --rerun-tasks --console=plain --no-daemon — 18/18 tasks succeeded
• JDK 11 and JDK 21: complete IMFCPLValidatorTest — 9/9 invocations passed in each environment
• git diff --check passed

The existing fork workflow run remains action_required with zero jobs, which indicates maintainer approval is still required before GitHub Actions can start; it is not a test failure.