Add opt-in multi-GPU sweep parallelism; fix GPU pinning and run dir races

For shape-heavy ops the autotune sweep dominates wall time, not the
measurement. _parallel_warmup_plugin shards the sweep across N GPUs and
hands the merged configs to the normal single-GPU serial measurement, so
timing stays comparable. Activated by PARALLEL_WARMUP_GPUS=N (silent
no-op when unset, so it can stay in the plugin list); output layout,
REPLAY_FROM usage and the latency table are unchanged.

Measurement is deliberately not parallelized: N processes saturating one
box couple through the power/thermal budget, so per-card latency gets
dragged by its neighbours by an amount that does not reproduce. Config
keys whose winner differs across shards are counted and reported as a
WARNING -- that count is how much to trust the run.

Fixes found while auditing:

- Shards no longer overwrite CUDA_VISIBLE_DEVICES with a bare shard
  index, which a caller who had selected idle cards (e.g. 6,7) would see
  re-interpreted as absolute ids 0,1 -- silently benchmarking on the busy
  cards they were avoiding.
- Interrupting the sweep no longer leaves N subprocesses holding GPUs;
  children are terminated and reaped before the exception propagates
  (BaseException, since KeyboardInterrupt is not an Exception).
- run_pytest.sh claims its output dir with a bare mkdir and retreats to
  a -2/-3 suffix on collision. The second-resolution timestamp meant two
  concurrent runs shared one directory and overwrote each other.
- Replay eviction failures now emit a distinct compile_*_no_evict marker.
  LibTuner.cache is a sqlite-backed ConfigCache with no __delitem__, so
  the bad config could not be dropped and the retry re-read it, while the
  log still claimed a clean fallback to live autotune.

Also trims comment density in both shell scripts and reworks the README:
promotes the parallel and cudagraph sections out from under the A/B flow,
groups the env table by purpose, documents that two record-mode runs are
not comparable, and marks ab_fold_test.sh as a caliber example whose
switch upstream has already removed.

ab_fold_test.sh

@@ -1,17 +1,19 @@
 #!/bin/bash
-# A/B 测试参考示例：FLAGGEMS_MXFP4_FOLDSCALE（fold_scale）开关验证
-# （4 个 DeepSeek-V4-Flash trace yaml × FOLD=0/1 共 8 轮）。
-# 口径：USE_FLAGTUNE=0；FOLD=0（基线）先跑并 record autotune config，FOLD=1 用
-# REPLAY_FROM 回放同一套 config（两侧同配置比 wall-time）。
-# 对比其它开关/算子时，改循环变量与环境变量即可复用同一口径。
-# 全部输出汇总到 runs/ab_fold_<时间戳>.log；各轮 runs/<op>_<时间戳>/ 产物照常存档。
-# 用法：bash ab_fold_test.sh
+# A/B 对比口径示例：4 个 trace shape 集 × 开关 0/1，共 8 轮。
+#
+# 口径：每个 shape 集先跑基线（record autotune config），再用 REPLAY_FROM 回放
+# 同一套 config 跑对照侧，两侧只差被测开关。对比其它开关/算子时改循环变量即可。
+#
+# 注意：对比轴 FLAGGEMS_MXFP4_FOLDSCALE 属于 FlagGems，已被上游移除。开关不存在
+# 时本脚本仍会跑完并输出完整表格，但两侧执行同一份代码——套用前先确认对比轴有效
+# （grep 该开关名于 $FLAGGEMS_DIR/src 应有命中）。详见 README。
+#
+# 输出：runs/ab_fold_<时间戳>.log（汇总）+ 各轮 runs/<op>_<时间戳>/
 set -uo pipefail
 
 cd "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
-# 终端颜色：下方 tee 管道会让 run_pytest.sh 侧检测不到 tty，在这里先判断并下传
-# （与 run_pytest.sh 自身的做法一致）；总 log 最后统一去色。
+# tee 管道会让子脚本检测不到 tty，故在此判断后下传（同 run_pytest.sh 的做法）
 if [[ -t 1 && -z "${NO_COLOR:-}" ]]; then
   export FLAGGEMS_PERF_COLOR=always
 fi
@@ -31,7 +33,7 @@ LOG="runs/ab_fold_$(date +%Y%m%d-%H%M%S).log"
   echo "########## ALL DONE ##########"
 } 2>&1 | tee "$LOG"
 
-# 总 log 去掉 ANSI 转义，保证 grep/diff 面对纯文本（终端输出保留颜色）
+# 去掉 ANSI 转义以便 grep/diff（终端输出保留颜色）
 sed -i -E $'s/\x1b\\[[0-9;]*[A-Za-z]//g' "$LOG"
 
 echo ">>> 总 log: $(pwd)/$LOG"