The terminal tool arms a heartbeat only after its spawn bookkeeping, and a
fast-starting process has already written its first lines by then.
arm_heartbeat reset the output baseline to the current total, so those lines
were in no heartbeat at all: the agent never saw them until completion.
Baseline is the spawn (field default 0). Every produced line now lands in
exactly one heartbeat regardless of how long the spawn->arm gap is.
CI: tests/tools/test_process_heartbeat.py FLAKY on main (run 36033989953),
beats read ["", "", "second\n", ""]. Reproduced locally with a 0.3 s gap
between spawn_local and arm_heartbeat: base loses "first", fixed keeps it.
e94fc4e26a