test(benchmark): keep the a871f8f engine as a fixed calibration engine

The differential check's frozen bundle will be rebuilt for #302; the
relative gate keeps timing the unchanged a871f8f bundle so its ratios
stay comparable with the recorded history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/benchmark/README.md

@@ -18,9 +18,11 @@ The relative gate replaces the 300-second acceptance protocol named in
 policy. This is an explicit policy change from #279/#285: runners differ in
 speed, so a fixed wall-clock limit was flaky.
 
-Before the protocol, in the same process, the spec times the frozen engine
-(`src/app/services/frozen-engine/leave-planner.core.frozen.mjs`, the engine at
-a871f8f) on the `two years` scenario, best of three, and prints the result.
+Before the protocol, in the same process, the spec times the calibration engine
+(`src/app/services/frozen-engine/leave-planner.core.calibration.mjs`, the engine at
+a871f8f) on the `two years` scenario, best of three, and prints the result. This
+bundle never changes, so ratios stay comparable with the recorded history; the
+differential check's frozen bundle can be rebuilt independently.
 Each scenario's budget is its multiplier times that calibration; the mean
 `planningMs` of its two measured repetitions must not exceed it. A scenario
 without a multiplier fails too. `#286` tightens the multipliers once the speed