bench: log every A/B cell as it finishes, and cap it with a timeout

The three markers the README requires be checked — AUTOTUNE_REPLAY_FALLBACK,
BENCHMARK_DIRECT_NO_CUDAGRAPH, BENCHMARK_CAPTURE_POISONED — were only
reachable by grepping run.log afterwards, which happens when someone
remembers to. The NV run that reproduced PR763 wrote a per-cell status row
instead; a 6-round campaign here had no equivalent, so a missing cell, a
stalled cell, or a marker count going from 0 to nonzero was invisible until
the batch ended.

run_cell now writes round/op/side/switch/rc/elapsed_s/success/failed plus the
three marker counts to <root>/status.tsv as each cell completes, and wraps the
cell in CELL_TIMEOUT (default 2400s, matching the NV run) so one hung cell
cannot hold the batch. rc 124/137 is called out by name.

grep -c prints 0 and exits 1 when it finds nothing, so each count swallows its
status; without that the bookkeeping itself would mark a clean cell failed.

README.md

@@ -231,6 +231,12 @@ SWITCH=FLAGTREE_MUSA_RLC_ENHANCE OPS="topk masked_select" ROUNDS=6   ZL_BACKEND=
 `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。
 
+每跑完一格往 `<root>/status.tsv` 追一行：`round op side switch rc elapsed_s success
+failed replay_fallback no_cudagraph capture_poisoned`。README 要求必查的三个标记就在
+这张表里，跑的时候就能看见它们从 0 变成非 0，不用等结束再 grep——事后 grep 只在有人
+想起来的时候才发生。每格另有 `CELL_TIMEOUT`（默认 2400 秒）兜底，超时记 `rc=124` 并打
+`!!! CELL_TIMEOUT`，不会让一格卡死拖住整批。
+
 第 1 轮 A/B 跑完后，脚本比两侧 `ttgir/` 的指纹（相对路径 + 内容）。**逐字节相同就
 跳过这个算子**，并打 `!!! SWITCH_NO_IR_DIFF op=... switch=...`：这个开关没改变该算子的
 生成代码，剩下 N-1 轮测的全是噪声，最后还会报成一个"无差异"的结论。先查开关名拼写