pty/run answers with the command's output; pin cloud9 with 9ns concurrency and E.BUSY

The answer is now the header line (exit N, or exit N cut M when only the
last 64 KiB were kept, or exit N cut when its start scrolled out of the
history), then what the command printed: the screen text between its C and
D marks, which the marks handler pins so scrolling keeps them.

Also from review: a plain /log open honours its offset until follow is
written, so tail -n and less work; whitespace-only run lines are refused.

cloud9 is pinned at 82d8152c: 9ns keeps up to 32 requests in flight per
mount so a waiting read no longer freezes the rest of it, and E.BUSY
replaces the errno pardes spelled locally.

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

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