A run whose output a redraw moved above its start answers cut, never an empty whole

pty/run reads a command's output between its C and D marks. When the
end landed at or before the start, it answered `exit 0` with no output,
as if nothing was printed. That happens when the shell redraws its prompt
over the output, as it can in a window two rows high (winsize 10 2). Now
only equal marks mean nothing was printed. An end at the top-left cell
or above the start says `cut`, since how much was printed is not known.

bash, fish, long lines and 3000-line outputs at winsize 10 2 all came
back whole on this build, so the dogfood's loss could not be reproduced
directly. The test drives the marks into the inverted case the loss
needs.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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