Fix _split_cells to handle non-unit width characters correctly

[nisha-muthurajan]

Fixes #3299

The previous implementation used a proportional heuristic to estimate the starting character position, which overshot for multi-cell characters like emoji. Replace with a linear scan that accumulates real cell widths.

Type of changes

-✅ Bug fix

• New feature
• Documentation / docstrings
• ✅ Tests
• Other

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• AI was used to generate this PR

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Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (see note about typos above).
• ✅ I've added tests for new code.
• ✅ I accept that @willmcgugan may be pedantic in the code review.

Description

Fixes #3299

Segment._split_cells used a proportional heuristic to guess the starting
character position:

pos = int((cut / cell_length) * len(text))

This overshot for multi-cell characters (emoji, CJK) because it assumed all
characters have equal width. The fallback loop then couldn't recover correctly,
producing wrong splits like ('🦊🦊 ', ' abcdef') instead of ('🦊 ', ' abcdef').

Fixed by replacing the heuristic + loop with a simple linear scan that
accumulates real cell widths character by character, stopping precisely at
the cut point.

Added a regression test test_split_cells_emoji covering the two examples
from the issue plus an exact-boundary case.