Covers the review's adversarial probes: expired-deadline greedy drain
coalesces jobs queued behind an in-flight generation (delay>0 no longer
degrades to sequential), submit-after-stop raises, stop() fails
still-queued futures without hanging, a dispatch exception fans out to
every future in the group, a crashed _run task fails in-flight and
queued futures and rejects later submits, the pipeline capability gate
rejects non-opted-in pipelines (DMD/causal always vetoed), pre-set
latents are never merged, and distinct opaque objects are conservatively
incompatible. Verified the deadline and gate tests fail on the pre-fix
scheduler.