fix: exclude wide characters that would exceed endColumn

[RyogaK]

Summary

Fixes #43

When endColumn falls in the middle of a wide character (CJK or emoji), sliceAnsi currently includes the character in full, causing the result to exceed the requested column range. This change excludes such characters so that stringWidth(result) <= endColumn - startColumn is always guaranteed.

Before

sliceAnsi('あいう', 0, 1) → 'あ' (width=2, exceeds end=1)
sliceAnsi('あいう', 0, 3) → 'あい' (width=4, exceeds end=3)

After

sliceAnsi('あいう', 0, 1) → '' (width=0 ≤ 1)
sliceAnsi('あいう', 0, 3) → 'あ' (width=2 ≤ 3)

Implementation

A single condition added to the isPastEnd check in the main loop: a non-continuation character token whose position + visibleWidth exceeds end is treated as past end. The rest of the existing logic (closing active styles, etc.) works as-is.

Discussion: wide emoji graphemes

This change also applies the same rule to wide emoji graphemes (e.g. regional-indicator flags 🇮🇱), updating the existing tests that previously expected "round up" behavior. I believe this is consistent — callers that specify a column range should be able to rely on the result not exceeding it — but I'd like to get your thoughts on whether wide emoji graphemes should be treated differently from CJK wide characters.

Test plan

• Added test case for CJK wide character boundary behavior ('あいう')
• Updated surrogate pair test to expect exclusion when exceeding boundary
• Updated regional-indicator flag tests for consistent boundary behavior
• All 89 tests pass

[RyogaK]

Another idea: we could add an option to control the behavior when the boundary falls inside a wide character, e.g.:

sliceAnsi('あいう', 0, 3);                                     // 'あい' (default, current behavior)
sliceAnsi('あいう', 0, 3, { wideCharBoundary: 'exclude' });    // 'あ'

This would provide a solution for downstream projects that have strict column-width constraints (like cli-truncate) without affecting existing projects that accept the current "round up" behavior.

Happy to implement this approach if you prefer it.