feat: add multi-battery monitoring

- Report battery data for individual devices
- Select a representative battery for legacy fields and alerts
- Average named battery data independently
- Display multiple batteries in system charts
- Add cross-platform coverage and transport tests

internal/records/records.go

@@ -186,6 +186,9 @@ func AverageSystemStatsSlice(records []system.Stats) system.Stats {
 
 	// necessary because uint8 is not big enough for the sum
 	batterySum := 0
+	batteryCount := 0
+	batterySums := make(map[string]uint64)
+	batteryCounts := make(map[string]uint64)
 	// accumulate per-core usage across records
 	var cpuCoresSums []uint64
 	// accumulate cpu breakdown [user, system, iowait, steal, idle]
@@ -232,8 +235,15 @@ func AverageSystemStatsSlice(records []system.Stats) system.Stats {
 		for i := range stats.DiskIoStats {
 			sum.DiskIoStats[i] += stats.DiskIoStats[i]
 		}
-		batterySum += int(stats.Battery[0])
-		sum.Battery[1] = stats.Battery[1]
+		if hasBattery(stats.Battery, stats.Batteries) {
+			batterySum += int(stats.Battery[0])
+			batteryCount++
+			sum.Battery[1] = stats.Battery[1]
+		}
+		for name, percent := range stats.Batteries {
+			batterySums[name] += uint64(percent)
+			batteryCounts[name]++
+		}
 
 		// accumulate per-core usage if present
 		if stats.CpuCoresUsage != nil {
@@ -379,7 +389,15 @@ func AverageSystemStatsSlice(records []system.Stats) system.Stats {
 	sum.LoadAvg[2] = twoDecimals(sum.LoadAvg[2] / count)
 	sum.Bandwidth[0] = sum.Bandwidth[0] / uint64(count)
 	sum.Bandwidth[1] = sum.Bandwidth[1] / uint64(count)
-	sum.Battery[0] = uint8(batterySum / int(count))
+	if batteryCount > 0 {
+		sum.Battery[0] = uint8(batterySum / batteryCount)
+	}
+	if len(batterySums) > 0 {
+		sum.Batteries = make(map[string]uint8, len(batterySums))
+		for name, total := range batterySums {
+			sum.Batteries[name] = uint8(total / batteryCounts[name])
+		}
+	}
 
 	// Average network interfaces
 	if sum.NetworkInterfaces != nil {
@@ -467,6 +485,10 @@ func AverageSystemStatsSlice(records []system.Stats) system.Stats {
 	return sum
 }
 
+func hasBattery(legacy [2]uint8, batteries map[string]uint8) bool {
+	return legacy != [2]uint8{} || len(batteries) > 0
+}
+
 // Calculate the average stats of a list of container_stats records
 func (rm *RecordManager) AverageContainerStats(db dbx.Builder, records RecordIds) []container.Stats {
 	allStats := make([][]container.Stats, 0, len(records))