The #426 fix moved waitUntilExit and its timeout onto the same global(qos:.utility)
queue. Under sustained load every utility worker blocked in waitUntilExit, so the
timeout could never be scheduled to kill them and the menubar wedged on Loading
forever (confirmed via sample after ~a week of soak). Await process.terminationHandler
(fires on a Foundation queue, blocks no worker) so the timeout always has a free
thread. Add an actor-based async semaphore capping concurrent CLI spawns at 6.
The menubar wedged on "Loading Today…" for hours after an idle period.
Root cause: DataClient.runCLI called the blocking process.waitUntilExit()
from an async function on Swift's cooperative thread pool. On a 16-core
machine, 16 concurrent slow `codeburn` subprocesses pinned all 16
cooperative threads inside waitUntilExit; the 45s timeout — itself a Task
on that same pool — could then never be scheduled to kill them, so the
deadlock was permanent. Confirmed via sample: 16/16 cooperative threads
parked in waitUntilExit. PR #412 (AppStore inFlightKeys bookkeeping) was a
layer above the OS-thread deadlock and could not fix it.
Move both blocking points off the cooperative pool: bridge waitUntilExit
through a global (overcommit) queue via a continuation, and drive the
timeout from a DispatchSource on a global queue so it fires even when the
pool is saturated. Extract runProcess for testability; add a concurrency +
timeout smoke test and an output/exit-code test.
