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

agent/battery/battery_test.go

@@ -0,0 +1,35 @@
+package battery
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPrimarySelection(t *testing.T) {
+	tests := []struct {
+		name string
+		bats []Battery
+		want string
+	}{
+		{"largest reported capacity", []Battery{{Name: "Small", FullChargeCapacity: 20, HasFullChargeCapacity: true, System: true}, {Name: "Large", FullChargeCapacity: 80, HasFullChargeCapacity: true}}, "Large"},
+		{"reported ranks over missing", []Battery{{Name: "Unknown", System: true}, {Name: "Known", FullChargeCapacity: 1, HasFullChargeCapacity: true}}, "Known"},
+		{"system wins capacity tie", []Battery{{Name: "Peripheral", FullChargeCapacity: 50, HasFullChargeCapacity: true}, {Name: "System", FullChargeCapacity: 50, HasFullChargeCapacity: true, System: true}}, "System"},
+		{"name resolves final tie", []Battery{{Name: "Zed"}, {Name: "Alpha"}}, "Alpha"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, ok := Primary(tt.bats)
+			require.True(t, ok)
+			assert.Equal(t, tt.want, got.Name)
+		})
+	}
+	_, ok := Primary(nil)
+	assert.False(t, ok)
+}
+
+func TestNormalizeBatteriesFallbackNames(t *testing.T) {
+	bats := normalizeBatteries([]Battery{{}, {}, {Name: "Mouse"}, {Name: "Mouse"}})
+	assert.Equal(t, []string{"Battery 1", "Battery 2", "Mouse", "Mouse (2)"}, []string{bats[0].Name, bats[1].Name, bats[2].Name, bats[3].Name})
+}