fix(ru): split sentence-final abbreviations before Cyrillic starts

[yisding]

Motivation

• A recent expansion of the Russian abbreviation list caused sentence-final abbreviations (e.g. др, ср, англ, рус, etc.) to unconditionally protect their trailing period and therefore merge sentences when the following sentence began with a Cyrillic uppercase token.
• The intent is to keep inline gloss/locale tags (e.g. англ. Moscow) joined while restoring genuine sentence boundaries when the next token is a new Cyrillic sentence.

Description

• Added import unicodedata and a SENTENCE_FINAL_ABBREVIATIONS set listing language tags and common abbreviations that may legitimately end a sentence in Russian inside sentencesplit/lang/russian.py.
• Implemented a helper _starts_with_cyrillic_upper and changed replace_period_of_abbr to use a conditional regex replacement that restores a real period when the following non-space character is a Cyrillic uppercase start and otherwise protects the period with the non-boundary placeholder; this preserves the previous protection for Latin-uppercase glosses and lowercase continuations.
• Added regression tests in tests/lang/test_russian.py covering both split-before-Cyrillic cases (e.g. Это литература и др. Она важна.) and preserved join cases (e.g. Он читал англ. Moscow.).

Testing

• Ran uv run pytest tests/lang/test_russian.py --color yes and the Russian tests passed.
• Ran the full suite with uv run pytest --cov=sentencesplit tests/ --color yes and all tests passed (1703 passed, 8 xfailed in the run recorded here).
• Ran code quality commands uv run ruff check . and uv run ruff format --check . which succeeded, and performed a manual PoC using sentencesplit.Segmenter(language='ru') to verify the reported examples split correctly.

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