fix: guard IndexError in syllable separate() and remove debug prints

[mkpoli]

• import vibrato was unguarded at module level, crashing if the package wasn't installed
• Class annotations MeCab.Tagger and janome.tokenizer.Tokenizer were evaluated at class-def time without from __future__ import annotations

Added from __future__ import annotations, wrapped vibrato import in try/except, and added _has_vibrato guard to VibratoTokenizer.__init__ matching the existing MeCab/Janome pattern.

[coderabbitai]

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⚙️ Run configuration

Configuration used: defaults

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Run ID: 87bd3bae-3da6-4716-9f37-dc76bd7566f0

📥 Commits

Reviewing files that changed from the base of the PR and between be58ca7 and 89c7f55.

📒 Files selected for processing (2)

• src/ainconv/conversion/katakana.py
• src/ainconv/syllable.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/syllable-index-guard

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