"Head" semantics: keep what is already buffered and discard new lines.

Ring-buffer / "tail" semantics: discard the oldest retained line so the most recent output survives.

Fail-loud ceiling: once the cap would be exceeded the run errors with `ProcessError.OutputTooLarge` rather than silently dropping. Output exactly equal to a configured limit is retained; the error is raised on the next line/byte that would push past it, not on reaching the limit exactly. The pipe is still drained (the child never blocks); excess lines are counted but not retained. With no cap set (`MaxLines = None` and `MaxBytes = None`) there is no ceiling to cross, so `Error` behaves like the other overflow modes on an unbounded buffer: it retains everything and never trips `OutputTooLarge`.