CMD_STOP for factory buildqueue

For native canceling. Probably a separate command. An alternative would be to make the regular stop command end up there with some opt/modifier.

The issue identifies CMD_STOP and factory buildqueue behavior but names no files, tests, or entry points. First clarify whether native canceling requires a separate command or a stop modifier, then trace the relevant command and build-queue handling; done means the chosen cancellation behavior is implemented and covered by appropriate tests.

Written by the indexing model from the issue text.