v1.3.0 — Sentence-Pair Cross-Encoder

Sentence-Pair Cross-Encoder. A joint-encoding pair head (SentencePairClassifier,
num_classes for rerank/gate/NLI), pair collation, encoder segment support, and
special-token reservation. Additive and backward-compatible.

Added

• SentencePairClassifier (polaris.models): a cross-encoder that joint-encodes
  [CLS] a [SEP] b [SEP], pools ("cls" or "mean"), and scores with a linear
  head — num_classes=1 (ranking score), 2 (gate), 3 (NLI). Shares the trunk,
  so it initializes from a pretrained MLM via transfer_encoder_to.
• PairBatch / collate_pairs (polaris.collation): pack (text_a, text_b, label)
  triples into [CLS] a [SEP] b [SEP] with segment ids and longest-first truncation.
• cls_token / sep_token on Vocabulary (with cls_id / sep_id),
  build_vocabulary, and train_bpe (which also gains mask_token) — reserved
  first and contiguously so adding a special never renumbers corpus tokens.
• The build_model factory now recognizes the "pair_classifier" model type.

Changed

• TransformerEncoder.forward gains an optional token_type_ids argument; a
  zero-initialized segment embedding is added only when it is given, so existing
  single-segment call sites are byte-identical. Additive, backward-compatible.