feat: repeat an A/B over rounds and report whether the verdict holds

A single pair cannot say whether its own verdict is stable, and on this device
that is not a theoretical gap. Measured on an S5000, topk eliminated 32
convert_layout ops yet 17 of its 42 shapes changed the SIGN of their delta
between two equally valid runs. The cause is not the timer: graph capture and
direct timing agree to 1.00x on that operator, and the verdict moved a median of
1.1pp between them, inside the 1.7pp noise floor a same-timer control
established. The deltas are simply near zero, and one measurement cannot orient
them.

ab_repeat.sh runs an A/B over N rounds in independent processes. Round 1's A
side records the autotune configs and every later run -- including A's own later
rounds -- replays that one set, so configs are pinned across rounds and sides and
only the switch under test and measurement noise remain. Round order alternates
to cancel one-directional drift. An operator with a failed round gets no report,
since a report missing rounds reads exactly like a complete one.

batch_summary --ab-rounds aggregates <root>/r<k>_<A|B>/run.log and separates the
two dispersions, which answer different questions and were previously conflated:
the spread within one side's samples says how noisy the machine was, while the
spread of the per-round deltas says whether the verdict holds. Shapes whose
delta changes sign across rounds are counted and called out, because their
direction is not a conclusion at any timer.

The path is already validated on NVIDIA: six rounds narrowed router_gemm to
-16.6% ~ -16.2% with 0.4% side noise, which is what turned a single pair into a
statement.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -162,6 +162,38 @@ benchmark 目录，注册 `--level`/`--mode` 的 `conftest.py` 就没被加载
 只校验节点的文件那一半；`::test_<fn>` 交给 pytest，函数不存在时它自己会清楚地报
 `not found: ...`。
 
+### 多轮重复：ab_repeat.sh 与 --ab-rounds
+
+单对观测无法说明自己稳不稳。S5000 实测 `topk` 消除了 32 个 `convert_layout`，
+42 个 shape 里却有 17 个在两次同等有效的运行之间**改变了 delta 的符号**——那不是
+计时口径问题（图捕获与 direct 的延迟比是 1.00x），是收益本身落在零附近、单次
+测量给不出方向。
+
+```bash
+SWITCH=FLAGTREE_MUSA_RLC_ENHANCE OPS="topk masked_select" ROUNDS=6   ZL_BACKEND=musa FLAGGEMS_DIR=/path/to/FlagGems bash ab_repeat.sh
+```
+
+口径：第 1 轮 A 侧 record config，此后每一次运行（含 A 侧的后续轮次）都
+`REPLAY_FROM` 那一份，config 因此跨轮跨侧全部锁死；逐轮交替先后顺序抵消单向漂移。
+有轮次失败的算子不出报告——缺轮的报告会让人误以为是完整的 N 轮。
+
+报告分开给两种离散度，它们回答不同的问题：
+
+| 指标 | 含义 |
+|---|---|
+| 逐轮区间 | N 轮各自 delta 的 min~max，说明**判定**稳不稳 |
+| 单侧噪声 | 同一侧 N 次延迟的极差，说明**机器**吵不吵 |
+| 符号不稳行数 | delta 在轮次间改变符号的 shape 数，这些行的方向不可结论 |
+
+已有产物可以单独重出报告，目录形态是 `<root>/r<k>_<A|B>/run.log`：
+
+```bash
+python batch_summary.py --ab-rounds runs/ab_repeat_XXX/topk --label topk
+```
+
+NV 上验证过这条路径：`router_gemm` 六轮把逐轮区间收窄到 -16.6% ~ -16.2%、单侧
+噪声 0.4%，把单对观测升级成了结论。
+
 ### ab_fold_test.sh / ab_rlc_test.sh：A/B 测试参考示例
 
 `ab_fold_test.sh` 是上述 A/B 流程的现成参考：以 FlagGems `FLAGGEMS_MXFP4_FOLDSCALE`（fold_scale 优化）开关为对比对象，对 4 个 DeepSeek-V4-Flash 真实 trace shape 集（`shape/DeepSeek-V4-Flash-p{1024,4096,32768,65536}d1024.yaml`，随仓库提供）各跑一对 FOLD=0（基线，record）→ FOLD=1（replay 同 config），共 8 轮：