tools/ab: reusable A/B runner + analyzer (frozen-HEAD build, exact permutation test, liveness check)

tools/ab/arms.example.txt

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+# A/B arm file — one arm per line. Passed to ab_run.sh via --arms.
+#
+# Format:
+#     name | ENV_VAR=value ENV_VAR2=value2 | optional label
+#
+#   * `name` must be unique; it becomes <outdir>/<name>/.
+#   * the env part is exported into the battle process (so the bot inherits it);
+#     ab_analyze.py verifies each pair shows up in the bot's boot env report.
+#   * the first arm is the default reference for the permutation test.
+#   * `name` alone, or `name |`, is a control arm with no env override.
+#   * blank lines and lines starting with `#` are ignored.
+
+control | | unmodified power policy (TR_POWER_ENERGY_MIN default 0.5)
+pmin100 | TR_POWER_ENERGY_MIN=1.0 | never fire below power 1.0
+pmin075 | TR_POWER_ENERGY_MIN=0.75 | never fire below power 0.75