fix: resolve chunker ruff regressions

[haosenwang1018]

Summary

• remove the unused loop index when emitting sub-chunks from oversized single-line input
• replace fullwidth punctuation literals in the sentence-splitting regex with Unicode escapes
• restore a green ruff run on the current main branch without changing chunking behavior

Testing

• uv run ruff check src/memsearch/chunker.py
• uv run ruff format --check src/memsearch/chunker.py
• uv run python -m pytest tests/test_chunker.py -q