Add package-level annotations and Javadoc

[andymarek]

This PR adds comprehensive package-level documentation and @NullMarked annotations across all packages in the CRF library. The documentation includes usage examples, API references, and configuration guides to help developers understand and use the library effectively.

Changes

• Package-level null safety: Added package-info.java files with @NullMarked annotations to all 8 packages (6 in core module, 2 in mallet module), establishing null safety at the package level

• Core module documentation:

  • org.coordinatekit.crf.core - Documented core abstractions (TagProvider, Sequence, InputSequence, PositionedToken)
  • org.coordinatekit.crf.core.preprocessing - Listed all 8 FeatureExtractor implementations with an example showing how to combine extractors using CompositeFeatureExtractor
  • org.coordinatekit.crf.core.tag - Documented tagging result types (CrfTagger, TaggedSequence, TaggedPositionedToken, TagScore)
  • org.coordinatekit.crf.core.io - Documented training data I/O and XML format
  • org.coordinatekit.crf.core.train - Documented training interface
  • org.coordinatekit.crf.core.util - Documented utility classes

• Mallet module documentation:

  • org.coordinatekit.crf.mallet.train - Added complete example showing MalletCrfTrainer configuration including ModelOutputConfiguration and ConllOutputConfiguration, plus documentation of all configuration options
  • org.coordinatekit.crf.mallet.tag - Added example showing how to create a tagger, tag input, and process results with tag scores

• Javadoc improvements: Added JSpecify API link to the aggregateJavadoc task so annotations like @NullMarked link to their documentation

Testing

• Execute GitHub Actions

[github-actions]

Code Coverage

Overall Project	98.37%	🍏

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