fix(lookahead): ignore boundary zero-width characters

[yisding]

Motivation

• A trailing-remainder span for zero-width-only input made comparison_segments non-empty while the public segment() result was empty, causing should_wait_for_more() / segment_with_lookahead() to incorrectly return True for invisible text and make streaming callers wait.

Description

• Strip boundary zero-width characters via _strip_zero_width before terminal-text/ punctuation checks in _wait_for_last_segment() while preserving the original comparison_segments/span values, and add test_segment_with_lookahead_ignores_zero_width_only_input to tests/test_segmenter.py to cover the regression.

Testing

• Ran uv run pytest tests/test_segmenter.py -q and the targeted tests passed.
• Ran the full suite with uv run pytest --cov=sentencesplit tests/ --color yes and all tests passed (1698 passed, 8 xfailed).
• Ran lint/format checks with uv run ruff check . and uv run ruff format --check . which succeeded.

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