Calibrate battery ADC and document LiPo curve

- Add BATTERY_CAL config and debug logging for raw ADC samples
- Use LiPo voltage curve (4.2V full, 2.9V empty) for % mapping
- Document battery calibration, curve, and debug output in README

src/power_manager.cpp

@@ -6,7 +6,6 @@
 #include <esp_sleep.h>
 
 static constexpr float BATTERY_DIVIDER = 2.0f;
-static constexpr float BATTERY_CAL = 1.0f;
 static constexpr float ADC_REF_V = 3.3f;
 
 void power_sender_init() {
@@ -35,25 +34,79 @@ void power_configure_unused_pins_sender() {
 
 void read_battery(MeterData &data) {
   uint32_t sum = 0;
+  uint16_t samples[5] = {};
   for (uint8_t i = 0; i < 5; ++i) {
-    sum += analogRead(PIN_BAT_ADC);
+    samples[i] = analogRead(PIN_BAT_ADC);
+    sum += samples[i];
   }
   float avg = static_cast<float>(sum) / 5.0f;
   float v = (avg / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
+  if (SERIAL_DEBUG_MODE) {
+    Serial.printf("bat_adc: %u %u %u %u %u avg=%.1f v=%.3f\n",
+                  samples[0], samples[1], samples[2], samples[3], samples[4],
+                  static_cast<double>(avg), static_cast<double>(v));
+  }
 
   data.battery_voltage_v = v;
   data.battery_percent = battery_percent_from_voltage(v);
 }
 
 uint8_t battery_percent_from_voltage(float voltage_v) {
-  float pct = (voltage_v - 3.0f) / (4.2f - 3.0f) * 100.0f;
-  if (pct < 0.0f) {
-    pct = 0.0f;
+  if (isnan(voltage_v)) {
+    return 0;
   }
-  if (pct > 100.0f) {
-    pct = 100.0f;
+  struct LutPoint {
+    float v;
+    uint8_t pct;
+  };
+  static const LutPoint kCurve[] = {
+      {4.20f, 100},
+      {4.15f, 95},
+      {4.11f, 90},
+      {4.08f, 85},
+      {4.02f, 80},
+      {3.98f, 75},
+      {3.95f, 70},
+      {3.91f, 60},
+      {3.87f, 50},
+      {3.85f, 45},
+      {3.84f, 40},
+      {3.82f, 35},
+      {3.80f, 30},
+      {3.77f, 25},
+      {3.75f, 20},
+      {3.73f, 15},
+      {3.70f, 10},
+      {3.65f, 5},
+      {3.60f, 2},
+      {2.90f, 0},
+  };
+  if (voltage_v >= kCurve[0].v) {
+    return kCurve[0].pct;
   }
-  return static_cast<uint8_t>(pct + 0.5f);
+  if (voltage_v <= kCurve[sizeof(kCurve) / sizeof(kCurve[0]) - 1].v) {
+    return 0;
+  }
+  for (size_t i = 0; i + 1 < sizeof(kCurve) / sizeof(kCurve[0]); ++i) {
+    const LutPoint &hi = kCurve[i];
+    const LutPoint &lo = kCurve[i + 1];
+    if (voltage_v <= hi.v && voltage_v >= lo.v) {
+      float span = hi.v - lo.v;
+      if (span <= 0.0f) {
+        return lo.pct;
+      }
+      float t = (voltage_v - lo.v) / span;
+      float pct = lo.pct + t * (hi.pct - lo.pct);
+      if (pct < 0.0f) {
+        pct = 0.0f;
+      }
+      if (pct > 100.0f) {
+        pct = 100.0f;
+      }
+      return static_cast<uint8_t>(pct + 0.5f);
+    }
+  }
+  return 0;
 }
 
 void light_sleep_ms(uint32_t ms) {