Merge consecutive same-phase laps before computing the average reference line

A cool-down (or warm-up) split across two or more laps was drawing each
lap's own average as a separate flat segment, so a workout with a 2-lap
cool-down showed two different "cool-down averages" back to back instead
of one. Consecutive laps sharing an IntensityType are now merged into a
single phase segment with one duration-weighted average across all of them.

frontend/src/components/charts/ExpectedVsActualChart.tsx

@@ -177,6 +177,45 @@ function lapWindowAt(t: number, windows: LapWindow[]): LapWindow | undefined {
   return windows.find((w) => t >= w.start && t < w.end) ?? windows[windows.length - 1];
 }
 
+interface PhaseSegment {
+  start: number;
+  end: number;
+  intensityType: string;
+  avgPace: number | null;
+  avgHR: number | null;
+}
+
+// Merges consecutive laps of the same IntensityType into one segment (e.g. a
+// two-lap cool-down becomes one), with a single duration-weighted average
+// across all of its laps -- otherwise each lap would draw its own average
+// line, showing e.g. two different "cool-down averages" back to back where
+// there's really just one cool-down.
+function buildPhaseSegments(windows: LapWindow[]): PhaseSegment[] {
+  const segments: PhaseSegment[] = [];
+  let group: LapWindow[] = [];
+  function flush() {
+    if (group.length === 0) return;
+    segments.push({
+      start: group[0].start,
+      end: group[group.length - 1].end,
+      intensityType: group[0].intensityType,
+      avgPace: weightedMean(group.map((w) => [w.avgPace, w.end - w.start])),
+      avgHR: weightedMean(group.map((w) => [w.avgHR, w.end - w.start])),
+    });
+    group = [];
+  }
+  for (const w of windows) {
+    if (group.length > 0 && group[group.length - 1].intensityType !== w.intensityType) flush();
+    group.push(w);
+  }
+  flush();
+  return segments;
+}
+
+function phaseSegmentAt(t: number, segments: PhaseSegment[]): PhaseSegment | undefined {
+  return segments.find((s) => t >= s.start && t < s.end) ?? segments[segments.length - 1];
+}
+
 function rangeTooltipFormatter(rangeLabel: string, formatValue: (v: number) => string) {
   return (value: unknown, name: unknown): [string, string] => {
     if (Array.isArray(value)) return [`${formatValue(value[0])}–${formatValue(value[1])}`, rangeLabel];
@@ -223,10 +262,12 @@ export function ExpectedVsActualChart({ laps, samples }: { laps: Lap[]; samples:
 
   const lapWindows = buildLapWindows(laps);
   const lapTotalMin = lapWindows[lapWindows.length - 1].end;
+  const phaseSegments = buildPhaseSegments(lapWindows);
 
-  function phaseAvg(w: LapWindow | undefined, metric: "avgPace" | "avgHR"): number | null {
-    if (!w || (w.intensityType !== "WARMUP" && w.intensityType !== "COOLDOWN")) return null;
-    return w[metric];
+  function phaseAvg(t: number, metric: "avgPace" | "avgHR"): number | null {
+    const seg = phaseSegmentAt(t, phaseSegments);
+    if (!seg || (seg.intensityType !== "WARMUP" && seg.intensityType !== "COOLDOWN")) return null;
+    return seg[metric];
   }
 
   let points: Point[];
@@ -239,10 +280,10 @@ export function ExpectedVsActualChart({ laps, samples }: { laps: Lap[]; samples:
         t,
         actualPace: paceSecPerKm(s.SpeedMps),
         targetPaceRange: w?.targetPaceRange ?? null,
-        phaseAvgPace: phaseAvg(w, "avgPace"),
+        phaseAvgPace: phaseAvg(t, "avgPace"),
         actualHR: s.HeartRate,
         targetHRRange: w?.targetHRRange ?? null,
-        phaseAvgHR: phaseAvg(w, "avgHR"),
+        phaseAvgHR: phaseAvg(t, "avgHR"),
       };
     });
     elapsedMin = Math.max(lapTotalMin, points[points.length - 1].t);
@@ -253,10 +294,10 @@ export function ExpectedVsActualChart({ laps, samples }: { laps: Lap[]; samples:
       t: w.start,
       actualPace: null,
       targetPaceRange: w.targetPaceRange,
-      phaseAvgPace: phaseAvg(w, "avgPace"),
+      phaseAvgPace: phaseAvg(w.start, "avgPace"),
       actualHR: null,
       targetHRRange: w.targetHRRange,
-      phaseAvgHR: phaseAvg(w, "avgHR"),
+      phaseAvgHR: phaseAvg(w.start, "avgHR"),
     }));
     points.push({ ...points[points.length - 1], t: lapTotalMin });
     elapsedMin = lapTotalMin;