v0.6.1 flush: memcmp finds a row's first and last changed cells

The whole-row memcmp settled an unchanged row, but on a changed row the cell
loop then read again the prefix memcmp had already compared, and the trailing
trim walked unchanged cells one by one. Chunks of eight cells now find the
first and last changed cell, so no cell is compared twice and only the span
between them is scanned cell by cell. Output is byte-identical.

Measured on frames captured from vif at 60 fps (median of 15, one core):
vanguard 120x40 73.7 -> 66.6 us a frame (-10%), vanguard 240x66
158.8 -> 131.8 (-17%), main with the gun firing 40.0 -> 34.1 (-15%), main
idle 20.3 -> 14.0 (-31%). Dropping the row compare instead costs +15% to
+120% on the same frames.

README.md

@@ -45,7 +45,9 @@ behaves as `New`'s Terminal does, the writer goroutine included; `New` is
 The application owns a flat `[]Cell` buffer (row-major, `cells[y*width+x]`) and
 passes it to `Flush`. The output buffer diffs against the previously flushed frame:
 
-- Rows are scanned with early termination (trailing unchanged cells skipped).
+- A row's first and last changed cells are found by memcmp, eight cells a compare:
+  an unchanged row costs those compares alone, and only the cells between are
+  scanned one by one.
 - Cursor moves are emitted only when the write position is non-contiguous.
 - SGR state (fg, bg, attributes) is coalesced across cells; redundant sequences
   are suppressed.