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>

src/app/services/frozen-engine/leave-planner.core.calibration.d.mts

@@ -0,0 +1,22 @@
+import type {
+	PlanningArtifactsCallback,
+	PlanningCheckpoint,
+	PlanningExplanationCallback,
+	PlanningOutcome,
+	PlanningOutcomeCallback,
+} from '../leave-planner.core';
+
+export function calculatePlanningInternal(
+	request: unknown,
+	checkpointCallback?: undefined,
+	artifactsCallback?: PlanningArtifactsCallback,
+): PlanningOutcome;
+
+export function calculatePlanningAsyncInternal(
+	request: unknown,
+	checkpointCallback?: PlanningCheckpoint,
+	artifactsCallback?: PlanningArtifactsCallback,
+	outcomeCallback?: PlanningOutcomeCallback,
+	explanationCallback?: PlanningExplanationCallback,
+	isCancelled?: () => boolean,
+): Promise<PlanningOutcome>;