Mechanism: the detached build writer in the two "keeps custody until a
detached writer is gone" tests rewrote its heartbeat file in place with
pathlib.write_text, which opens with O_TRUNC and then writes. Custody
SIGKILLs that writer at an arbitrary instant; when the kill lands between
the truncating open and the write, `beat` is left empty and the teardown's
`beat.read_text().split()[0]` raises IndexError (suppress only covered
ProcessLookupError/ValueError). CI hit it on
test_a_ctrl_c_keeps_custody_until_a_detached_writer_is_gone[verbose-slow-cleanup]
(job 112764854735).
Repro: scratch copy of the writer with a 0.1 s sleep between open() and
write() -> both ctrl-c cases and the group-kill sibling fail with
IndexError at the teardown read on every run (5/5).
Fix: one shared _heartbeat_writer helper writes each beat to `<beat>.tmp`
and os.replace()s it into place, so `beat` always holds a complete
"<pid> <n>" once it exists. With the same injection moved onto the temp
write, all three cases pass 5/5. The ValueError suppress that papered over
a partial read is dropped.
464572083b