Review round 4 (codex R3): two concurrent `start_timer` calls for the same
attempt both passed the "no timer" check, both spawned, and the pidfile
tracked only one - the untracked timer could still publish a retained
`running` after `completed`. A per-job flock (`<job>.heartbeat.lock`,
separate from the registry/seq lock so in-flight beats can still take
their seq) now makes observe/drain/spawn/publish-pid one ownership
operation; the terminal stop takes the same lock; a start that finds the
attempt ended (terminal record or raised barrier) does not arm; the timer
removes its own pidfile only under the lock so a replacement's pid is never
deleted. Regressions: concurrent starts leave exactly one tracked owner that
the terminal stop drains; a start after the barrier arms nothing.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff
GodopuandClaude Fable 5.1
2b1d590010