In #registerSpawnJob the markRunning()/reportProgress() calls sat between
semaphore.acquire() and the try whose finally releases the slot. If progress
reporting threw there, the acquired task.maxConcurrency slot leaked and
permanently shrank subagent concurrency. Move those statements inside the try
so finally always releases. The abort-before-execution branch is unchanged
(releases once and throws before the try is entered — not a double release).
Refs #3464
can1357
06ca03dbd8