Fix consumption tracking while charging

app/src/main/java/com/evsuite/abrp/KahanSum.java

@@ -0,0 +1,38 @@
+package com.evsuite.abrp;
+
+/**
+ * Neumaier–Kahan compensated summation.
+ *
+ * Consumption is integrated from thousands of sub-watt-hour slices per drive. A plain
+ * {@code sum += x} loses the low bits of every one of them, and the error is one-signed,
+ * so it accumulates rather than cancelling. Ported from DriveHub_Dort, where this is what
+ * keeps the lifetime counter honest over months.
+ */
+final class KahanSum {
+
+    private double sum;
+    private double compensation;
+
+    void add(double input) {
+        double y = input - compensation;
+        double t = sum + y;
+        compensation = (t - sum) - y;
+        sum = t;
+    }
+
+    double get() {
+        return sum;
+    }
+
+    /** Clears the total and the compensation term. */
+    void reset() {
+        sum = 0d;
+        compensation = 0d;
+    }
+
+    /** Seeds the total from persistent storage; the compensation term starts clean. */
+    void setTotal(double total) {
+        sum = total;
+        compensation = 0d;
+    }
+}