fix(term): Measure truncation prefixes as whole strings

[JeanMertz]

truncate_to_width summed per-cluster display widths to find where to
cut a string before appending an ellipsis. That sum is wrong for
sequences that ligate or combine into something narrower than their
parts: Arabic Lam-Alef spans two clusters that sum to two columns but
render as one, and similar sequences elsewhere could overspend the
budget and cut earlier than necessary. The fix measures each candidate
prefix as a whole string instead of summing its clusters, so ligating
sequences are accounted for at their true rendered width.

This also fixes a zero-width edge case: a zero-column input (e.g. a
lone ZWJ) now survives a zero-column budget unchanged, since it
already fits, rather than being dropped for the ellipsis. Only input
wider than the budget is truncated.