Add Doxygen documentation to core classes and language models

[Copilot]

Added comprehensive Doxygen comments to 12 core header files to improve code comprehension for developers and AI tools analyzing the morphological analyzer architecture.

Language Model Layer

• LangModel.h - ILangModel interface, CRTP state base, void model stub
• Knlm.h - Kneser-Ney n-gram implementation with header/node structures
• SkipBigramModel.h - Long-distance dependency capture via skip-bigrams
• CoNgramModel.h - Neural embedding-based contextual n-grams with similarity queries

Morpheme Processing

• Joiner.h - Joiner/AutoJoiner for morpheme recombination with phonological rules
• WordDetector.h - Statistical word extraction using cohesion/branching entropy
• PatternMatcher.h - Match enum flags for URL/email/hashtag/mention recognition

Infrastructure

• FrozenTrie.h - Immutable trie with Aho-Corasick fail links for dictionary lookup
• ThreadPool.h - Work queue with thread-id-aware task dispatch
• Utils.h - UTF-8/16 conversion, Hangul syllable decomposition
• TagUtils.h - POS tag classification predicates and sequence scoring
• Mmap.h - Memory-mapped file I/O for model loading

Documentation follows existing Doxygen conventions with Korean explanations for domain-specific morphological concepts. All changes are comment-only with no functional impact.

Stats: 862 lines added across 12 files

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Original prompt

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This section details on the original issue you should resolve

<issue_title>코드 문서화 강화</issue_title>
<issue_description>전체 시스템의 코드 구조를 파악하고 핵심 클래스, 함수들에 Doxygen 주석으로 설명을 추가해줘.
내가 작성해놓은 Doxygen 주석들이 일부 있으니 이걸 참고하면 도움이 될거야. 작성해놓은 Doxygen 주석에서 누락된 애들도 많으니 이것도 보충해주면 좋고.
목표는 다른 사람 or AI 봇이 보았을때 코드 및 형태소 분석기의 동작을 잘 이해할 수 있도록 하는 거야.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 코드 문서화 강화 #234

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