fix(metrics): stabilize cpu and power sampling

audit/internal/platform/live_metrics_test.go

@@ -42,3 +42,53 @@ func TestCompactAmbientTempName(t *testing.T) {
 		t.Fatalf("got %q", got)
 	}
 }
+
+func TestCPULoadPctBetween(t *testing.T) {
+	tests := []struct {
+		name      string
+		prevTotal uint64
+		prevIdle  uint64
+		total     uint64
+		idle      uint64
+		want      float64
+	}{
+		{
+			name:      "busy half",
+			prevTotal: 100,
+			prevIdle:  40,
+			total:     200,
+			idle:      90,
+			want:      50,
+		},
+		{
+			name:      "fully busy",
+			prevTotal: 100,
+			prevIdle:  40,
+			total:     200,
+			idle:      40,
+			want:      100,
+		},
+		{
+			name:      "no progress",
+			prevTotal: 100,
+			prevIdle:  40,
+			total:     100,
+			idle:      40,
+			want:      0,
+		},
+		{
+			name:      "idle delta larger than total clamps to zero",
+			prevTotal: 100,
+			prevIdle:  40,
+			total:     200,
+			idle:      150,
+			want:      0,
+		},
+	}
+
+	for _, tc := range tests {
+		if got := cpuLoadPctBetween(tc.prevTotal, tc.prevIdle, tc.total, tc.idle); got != tc.want {
+			t.Fatalf("%s: cpuLoadPctBetween(...)=%v want %v", tc.name, got, tc.want)
+		}
+	}
+}