One job at a time, a 3D job waiting for the card held up another model's
job that could have run (seen on the PoC, BACKLOG H7). The leader's worker
claims jobs while fewer than eight run, passing over the models that have
one running, and starts each in its own goroutine; one ending wakes it for
the next. Lease and leader-only rules are as they were.
BACKLOG: H5–H7 marked, H8 (presets) and H9 (what the PoC taught) written.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>