perf(harness): warm the runner after open so the first measured command is clean

The first interaction after open/relaunch pays the one-time iOS XCUITest runner
startup (~10s+ cold) and a per-relaunch first-AX-query settle cost (~4s). That was
landing on the first measured command each round (snapshot -i), inflating it ~10x
vs the next snapshot. Run an untimed warmup snapshot -i after establishSession, after
each round's reset-open, and after every freshRoot relaunch, so no measured command
absorbs runner startup. Noted in the report header.

Author: thymikee

scripts/perf/harness.ts

@@ -65,10 +65,18 @@ function sampleError(r: ReturnType<typeof invokeCli>): Pick<Sample, 'errorCode'
   };
 }
 
+// The first interaction after open/relaunch pays the iOS XCUITest runner startup (~10s+ cold)
+// and a per-relaunch first-AX-query settle cost. Run an untimed throwaway interaction so that
+// cost is never attributed to a measured command.
+function warmRunner(ctx: IsolationContext): void {
+  invokeCli(['snapshot', '-i'], ctx.baseFlags);
+}
+
 function runStep(step: ScenarioStep, ctx: IsolationContext, round: number): Sample {
   // Untimed reset to root for steps whose precondition is a clean, top-of-list root.
   if (step.freshRoot) {
     invokeCli(['open', ctx.profile.appTarget, '--relaunch'], ctx.baseFlags);
+    warmRunner(ctx);
   }
   const r =
     step.execMode === 'standalone'
@@ -151,6 +159,8 @@ export function runScenario(ctx: IsolationContext, cfg: PerfConfig): Measurement
 
   const boot = bootOnce(ctx);
   const establish = establishSession(ctx);
+  // Absorb the one-time runner startup before any round so it isn't charged to a measurement.
+  warmRunner(ctx);
 
   // Android accessibility dumps time out while the UI is animating; disable animations
   // up front (untimed) so snapshot/get/is/fill can read an idle hierarchy.
@@ -166,6 +176,11 @@ export function runScenario(ctx: IsolationContext, cfg: PerfConfig): Measurement
     for (const step of steps) {
       const sample = runStep(step, ctx, round);
       if (measured) acc.get(step.label)!.push(sample);
+      // After the round's reset-open relaunch, warm the runner (untimed) so the first measured
+      // read (snapshot -i) doesn't pay the post-relaunch first-AX-query cost.
+      if (step.command === 'open' && step.execMode === 'standalone') {
+        warmRunner(ctx);
+      }
     }
   }
 

scripts/perf/report.ts

@@ -35,6 +35,8 @@ function toMarkdown(run: RunResult): string {
   lines.push('All times in milliseconds. `wall-clock` includes process spawn + socket overhead;');
   lines.push('`daemon` is the batch step round-trip (spawn overhead ≈ wall-median − daemon-median).');
   lines.push('`elements` = distinct interactive @eN refs in the snapshot payload (tree-size proxy).');
+  lines.push('An untimed warmup interaction runs after each open/relaunch, so measured commands');
+  lines.push('do not pay the one-time iOS-runner startup or post-relaunch first-AX-query cost.');
   lines.push('');
   lines.push('| command | cli | mode | n | wall min | wall median | wall p95 | wall max | daemon median | elements | notes |');
   lines.push('|---|---|---|---|---|---|---|---|---|---|---|');