Two defects made the earlier A/B numbers meaningless:
* Without `--stop-on-run-end` a session contained several runs, so runs could
not be attributed to an arm.
* The script then read "the last 2N runs", which silently compared runs from
earlier experiments rather than the two arms.
Pass `--stop-on-run-end`, and attribute each run record to its arm by diffing
the history directory around each session.
Also raise the step budget and warn on a thin sample: measured, a run still
going at step 600 produces NO run record, so a short budget biases the sample
toward runs that died early -- exactly the wrong bias for this question.
eval_batch.sh: N back-to-back sessions with per-session summaries.
ab_card_skip.sh: A/B the jev vs combined card-reward skip policy and
compare deck size and progress from the game's run history.
