Resolves conflicts with the #65 resize fix that landed first. Both branches
converged on the same resize-coalescing + display-width truncation fix;
took the perf branch's more detailed inline comments and combined the
transcript bench from #78 with the existing #65 resize regression tests.
Issue #78 baseline (release, 5000-cell synthetic transcript):
pure scroll, off=0 3549µs → 21µs (~150x)
pure scroll, off=2000 3303µs → 19µs (~170x)
streaming append 11.6ms → 3.4ms (~3.4x)
Hunter Bown
aa8d0dc73a