Status: Tighten ahocorasick-ner Integration

Checklist

Rename _AhocorasickNER import in lookup_ner.py; update all usages including string type annotation on line 67
Rename _AhocorasickNER import in locations_ner.py; update usage in _build_automaton
Add min_word_len: int = 5 (keyword-only) to AhocorasickAnnotatorWrapper.__init__; store and forward to tag()
Add LookUpNER.add_word(label, word) method with automaton rebuild
Update AhocorasickAnnotatorWrapper docstring with min_word_len param doc and dataset-loader example
Update docs/index.md AhocorasickAnnotatorWrapper section
Add tests: min_word_len forwarding (custom + default), LookUpNER.add_word, no-_AhocorasickNER-symbol check
Run uv run pytest test/ -v --cov=simple_NER --cov-report=term-missing; confirm 462+ pass, coverage ≥ 89%