Root cause: executeThread awaited gen.next() without racing against
the abort signal. When a workflow bundle awaited a long setTimeout
between yields, the engine could not respond to kill until the
Promise resolved — causing the kill test to flake when the thread
completed before kill arrived.
Fix: Promise.race gen.next() with an abort listener so kill takes
effect immediately, even mid-yield. Also move the bundle's delay
to after the first yield (between planner and coder) to ensure the
thread is killable while running.
Closes#209
xingyue
5b7c9b844b