OPENNLP-1885: Add SentencePiece subword add-on

[krickert]

Summary

• implements the OpenNLP SubwordTokenizer API with pure Java SentencePiece inference
• loads SentencePiece .model files and supports unigram and byte-pair models
• preserves exact UTF-16 spans into the caller's original text after model normalization
• exposes vocabulary lookup, piece scores, algorithm type, and aligned normalization
• adds generated test models, independent parity fixtures, usage documentation, and 66 tests

No runtime model, native library, or third-party training data is bundled.

This is the example add-on contribution described by #177.

Dependencies

• OPENNLP-1924: Prepare add-ons for 3.x contributions #177 prepared the add-ons canary, manual, and reactor
• OPENNLP-1885: Add subword API and WordPiece encoder, remove BertTokenizer opennlp#1165 supplies SubwordTokenizer, SubwordPiece, and the shared offset contract

The PR targets OPENNLP-1924-canary-addon. It remains a draft until the
core API dependency is available to the add-ons build.

Validation

• mvn clean verify -Dopennlp.forkCount=1: all seven reactor projects passed
• SentencePiece: 66 tests passed, with one opt-in real-model evaluation skipped
• mvn -N clean site: 9 HTML pages generated
• local links and fragments passed
• RAT reported zero unapproved files
• all six serializable implementation classes match their computed serialver values

[krickert]

So this is our "canary" PR that would go into our "addons" - of course we'll need to setup the infra for the whole addons, but this would be the impl for the OPENNLP-1885. This will unlock a lot of the embedding add-ons that are being proposed.