test(perf): refuse a J1 idle window that opened late after the settle point

Addresses review: the idle window opens in the settle timer's callback while
the settle point is that timer's deadline, so a late callback left ticks
uncounted between the two.

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

apps/electron-backend-e2e/src/performance/launch-journey-record.spec.ts

@@ -31,8 +31,8 @@ function measurement(
         firstCardPaintEpochMs: 2_650,
         idle: {
             domMutations: 12,
-            endEpochMs: 36_000.04,
-            startEpochMs: 6_000,
+            endEpochMs: 33_200.04,
+            startEpochMs: 3_200,
             status: 'done',
             ticks: 31,
         },
@@ -198,7 +198,7 @@ test('maps the probe, IPC capture and main counters to exact counters and spawn-
     assert.deepEqual(record.evidence['idle'], {
         domMutations: 12,
         durationMs: 30_000,
-        settledToIdleStartMs: 2_819.9,
+        settledToIdleStartMs: 19.9,
     });
 });
 
@@ -278,14 +278,32 @@ test('refuses a launch without a complete, on-time idle window after the settle
             ),
         /idle-before-settle/
     );
+    // The settle point is 3_180.06: a window opened 120 ms after it left
+    // ticks uncounted in between.
     assert.throws(
         () =>
-            toLaunchIterationRecord(0, false, withIdle({ endEpochMs: 35_000 })),
+            toLaunchIterationRecord(
+                0,
+                false,
+                withIdle({ endEpochMs: 33_300.1, startEpochMs: 3_300.1 })
+            ),
+        /idle-start-late/
+    );
+    assert.doesNotThrow(() =>
+        toLaunchIterationRecord(
+            0,
+            false,
+            withIdle({ endEpochMs: 33_250, startEpochMs: 3_250 })
+        )
+    );
+    assert.throws(
+        () =>
+            toLaunchIterationRecord(0, false, withIdle({ endEpochMs: 33_000 })),
         /idle-window-short/
     );
     assert.throws(
         () =>
-            toLaunchIterationRecord(0, false, withIdle({ endEpochMs: 37_500 })),
+            toLaunchIterationRecord(0, false, withIdle({ endEpochMs: 34_500 })),
         /idle-window-late/
     );
     assert.equal(
@@ -332,7 +350,7 @@ test('refuses a launch whose settle window did not end after the first-card cuto
     const capped = toLaunchIterationRecord(
         0,
         false,
-        withSettle({ epochMs: 5_650, status: 'cap' })
+        withSettle({ epochMs: 3_150, status: 'cap' })
     );
     assert.equal(
         (capped.evidence['settle'] as { reason: string }).reason,

apps/electron-backend-e2e/src/performance/launch-journey-record.ts

@@ -50,6 +50,14 @@ export const LAUNCH_JOURNEY_UNAVAILABLE_COUNTERS: Readonly<
  */
 export const LAUNCH_JOURNEY_IDLE_LATE_TOLERANCE_MS = 1_000;
 
+/**
+ * The idle window opens in the settle timer's callback, but the settle point
+ * is that timer's deadline. A callback that ran later than this left ticks
+ * between the two outside both windows, and means the page was still busy
+ * at the settle point, so the iteration is refused rather than undercounted.
+ */
+export const LAUNCH_JOURNEY_IDLE_START_TOLERANCE_MS = 100;
+
 export interface LaunchJourneyMeasurement {
     readonly electronVersion: string;
     readonly gate: JourneyRendererGateState;
@@ -230,6 +238,12 @@ function assertLaunchIdleWindow(
     if (idle.startEpochMs < settledEpochMs) {
         throw new Error('launch-journey-record-idle-before-settle');
     }
+    if (
+        idle.startEpochMs - settledEpochMs >
+        LAUNCH_JOURNEY_IDLE_START_TOLERANCE_MS
+    ) {
+        throw new Error('launch-journey-record-idle-start-late');
+    }
     const durationMs = idle.endEpochMs - idle.startEpochMs;
     // Timers may fire up to a millisecond early after clamping.
     if (durationMs < JOURNEY_IDLE_WINDOW_MS - 1) {