Eliminate resize hang on column change

[shellicar]

Summary

• Eliminate ~10ms render hang when terminal columns change
• Pre-compute line segments on append for arithmetic-based re-wrap on resize
• Add performance test asserting resize re-render at 10K lines completes under 2ms

Closes #166

[bananabot9000]

Pre-computing line segments on append to avoid re-running Intl.Segmenter on resize -- smart trade-off of memory for speed. The width-1 bulk slice optimisation is the real win: ASCII lines (vast majority of terminal output) become arithmetic-only on re-wrap.

One note for the future: lineSegments array grows with displayBuffer. If display buffer ever gets trimmed/rotated, segments need to stay in sync. Not a blocker -- just worth a comment.

10K lines from ~10ms to <2ms. Clean perf test with CI-aware thresholds.

LGTM 🍌