Enforce the parent-completion invariant at claim_task (the single
ready->running chokepoint) and re-gate unblock_task so blocked->ready
only fires when parents are done. Prevents child tasks from running
ahead of in-progress parents under the create-then-link race.
Also adds a stress test that races concurrent create+link against
hammered claim_task and asserts no child runs while any parent is undone.
Ref: kanban/boards/cookai/workspaces/t_a6acd07d/root-cause.md
Refs: t_8d6af9d6
