Follow-up on the parallel-dispatch decoupling: the sequential pass for
workdir/profile jobs still ran inline in the ticker thread, so a long
workdir/profile job reintroduced the exact starvation #37312 describes,
just for env-mutating jobs. And the MCP orphan sweep ran immediately
after dispatch in sync=False mode — before jobs finished — defeating its
own 'runs after every job' contract and racing jobs still spawning MCP
children.
- Sequential jobs now queue to a persistent single-thread cron-seq pool
(preserves one-at-a-time ordering across ticks, never blocks the tick).
- Same in-flight dedup guard now covers sequential jobs.
- MCP orphan sweep runs via a done-callback after the LAST dispatched job
completes in async mode; inline after as_completed in sync mode.
Verified E2E: tick(sync=False) returns in ~1ms with a 1.5s sequential job
in flight; sweep fires only after that job ends.
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