fix(compare): gap deltas beyond reference laps

frontend/src/lib/delta.ts

@@ -35,12 +35,12 @@ function buildAlignedCumulative(
   const length = Math.max(lapTimes.length, referenceLapTimes.length)
 
   for (let i = 0; i < length; i++) {
-    if (referenceLapTimes[i] === null) {
+    if (referenceLapTimes[i] == null) {
       cumulative.push(running)
       continue
     }
     const lap = lapTimes[i]
-    if (lap !== null) {
+    if (lap != null) {
       running += lap
     }
     cumulative.push(running)
@@ -84,7 +84,7 @@ export function computeCumulativeDeltas(
       const deltas: (number | null)[] = []
 
       for (let i = 0; i < lapCount; i++) {
-        if (refLapTimes[i] === null || driver.lapTimes[i] === null) {
+        if (refLapTimes[i] == null || driver.lapTimes[i] == null) {
           deltas.push(null)
           continue
         }

frontend/src/test/delta-time-graph.test.tsx

@@ -78,6 +78,23 @@ describe('computeCumulativeDeltas', () => {
     expect(result[0].deltas[2]).toBeCloseTo(-3)
   })
 
+  it('gaps challenger laps beyond a shorter reference series', () => {
+    const shortReference: DeltaSeries = {
+      label: 'VER',
+      color: '#3671C6',
+      lapTimes: [90, 91],
+    }
+    const longerChallenger: DeltaSeries = {
+      label: 'HAM',
+      color: '#E8002D',
+      lapTimes: [89, 92, 90],
+    }
+    const result = computeCumulativeDeltas([shortReference, longerChallenger])
+    expect(result[0].deltas[0]).toBeCloseTo(-1)
+    expect(result[0].deltas[1]).toBeCloseTo(0)
+    expect(result[0].deltas[2]).toBeNull()
+  })
+
   it('returns an empty array when only one series is provided', () => {
     expect(computeCumulativeDeltas([reference])).toEqual([])
   })