[codex] Fix matrix-arff release findings

[perNyfelt]

Summary

• Quote ARFF-sensitive nominal literals so values such as ?, empty strings, and %... round-trip as data instead of missing values or comments.
• Add shared strict DATE formatter handling for ARFF parsing and writing using Locale.ROOT and UTC.
• Fix the matrix-arff published SCM URL to point at the module on main.
• Add regression coverage for nominal sentinel round trips, invalid date rejection, and UTC date output.

Impact

This avoids lossy ARFF nominal serialization and makes DATE parsing fail on invalid dates instead of silently normalizing them. The POM metadata now points to the correct module location for release consumers.

Validation

• ./gradlew :matrix-arff:test
• ./gradlew :matrix-arff:build
• git diff --check