12-OBD-Speed: battery voltage arc and idle/screen-off mode

- ATRV every 2 s; voltage arc in the bottom gap (11-15 V) + number,
  colored by state (low / weak / resting / charging / overcharge)
- idle after Off/2/5/10 min without speed data (settings page, NVS):
  screen off, GIF paused, OBD polling stopped, ATRV every 30 s;
  wakes on touch (swallowed by a top-layer overlay) or > 13.2 V
- status pill moved up to make room for the voltage

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

12-OBD-Speed/src/obd_link.cpp

@@ -4,12 +4,20 @@
 #include <Preferences.h>
 
 #define RETRY_DELAY_MS 3000
+#define IDLE_RETRY_DELAY_MS 30000
 #define POLL_FAIL_LIMIT 3
+#define VOLTAGE_INTERVAL_MS 2000       // ATRV while driving
+#define IDLE_VOLTAGE_INTERVAL_MS 30000 // ATRV while idle (adapter-internal, doesn't wake the car's bus)
+#define WAKE_VOLTAGE 13.2f             // above this the alternator is charging: engine running
 
 // ---------- shared state (protected by `mtx`) ----------
 static SemaphoreHandle_t mtx;
 static volatile ObdState state = ObdState::NoDevice;
 static volatile int speed_kmh = -1;
+static volatile float voltage = -1;
+static volatile bool idle = false;
+static volatile uint32_t idle_timeout_ms = 2 * 60 * 1000;
+static volatile uint32_t last_data_ms = 0; // last valid speed (or wake-up)
 static String status_text = "starting";
 static String saved_addr, saved_name;
 static uint8_t saved_type = 0;
@@ -18,7 +26,7 @@ static uint32_t devices_version = 1;
 static String elm_rx; // bytes received from the ELM (notifications)
 
 // ---------- requests from the UI ----------
-static volatile bool req_select = false, req_forget = false;
+static volatile bool req_select = false, req_forget = false, req_wake = false;
 static volatile bool scan_wanted = false;
 static ObdDevInfo req_dev;
 
@@ -155,6 +163,23 @@ static int parse_speed(String reply)
   return (int)strtol(reply.substring(i + 4, i + 6).c_str(), nullptr, 16);
 }
 
+// "12.6V" -> 12.6, -1 if not a voltage
+static float parse_voltage(const String &reply)
+{
+  int v = reply.indexOf('V');
+  if (v <= 0)
+    return -1;
+  float f = reply.substring(0, v).toFloat();
+  return f > 0 ? f : -1;
+}
+
+static void read_voltage()
+{
+  float v = parse_voltage(elm_command("ATRV", 1500));
+  if (v > 0)
+    voltage = v;
+}
+
 // ---------- connection ----------
 static void disconnect()
 {
@@ -259,9 +284,28 @@ static void load_saved()
 static void worker_task(void *)
 {
   uint32_t next_attempt = 0;
+  uint32_t next_voltage = 0;
   int fails = 0;
+  last_data_ms = millis();
   for (;;)
   {
+    if (req_wake)
+    {
+      req_wake = false;
+      if (idle)
+        Serial.println("[obd] wake (touch)");
+      idle = false;
+      last_data_ms = millis();
+      next_attempt = 0;
+    }
+    // no speed data for a while (ignition off / adapter gone): go idle
+    if (!idle && idle_timeout_ms && millis() - last_data_ms > idle_timeout_ms)
+    {
+      idle = true;
+      speed_kmh = -1;
+      Serial.println("[obd] idle");
+    }
+
     // requests from the UI
     if (req_forget)
     {
@@ -296,6 +340,8 @@ static void worker_task(void *)
     if (saved_addr.length() == 0)
     {
       state = ObdState::NoDevice;
+      idle = false;
+      last_data_ms = millis(); // no idle while no adapter is chosen (user is setting up)
       set_status("No adapter - swipe left");
       vTaskDelay(pdMS_TO_TICKS(100));
       continue;
@@ -317,11 +363,36 @@ static void worker_task(void *)
       {
         disconnect();
         state = ObdState::Waiting;
-        next_attempt = millis() + RETRY_DELAY_MS;
+        next_attempt = millis() + (idle ? IDLE_RETRY_DELAY_MS : RETRY_DELAY_MS);
       }
+      next_voltage = 0;
       continue;
     }
 
+    if (idle)
+    {
+      // only the battery voltage now and then; an engine start (charging voltage) wakes us up
+      if ((int32_t)(millis() - next_voltage) >= 0)
+      {
+        next_voltage = millis() + IDLE_VOLTAGE_INTERVAL_MS;
+        read_voltage();
+        if (voltage > WAKE_VOLTAGE)
+        {
+          idle = false;
+          last_data_ms = millis();
+          Serial.printf("[obd] wake (%.1f V)\n", voltage);
+        }
+      }
+      vTaskDelay(pdMS_TO_TICKS(100));
+      continue;
+    }
+
+    if ((int32_t)(millis() - next_voltage) >= 0)
+    {
+      next_voltage = millis() + VOLTAGE_INTERVAL_MS;
+      read_voltage();
+    }
+
     // connected: poll the vehicle speed
     String reply = elm_command("010D", 1500);
     int v = parse_speed(reply);
@@ -329,6 +400,7 @@ static void worker_task(void *)
     if (v >= 0)
     {
       fails = 0;
+      last_data_ms = millis();
       set_status(saved_name.length() ? saved_name : saved_addr);
     }
     else if (reply.length() == 0)
@@ -365,6 +437,10 @@ void obd_begin()
 
 ObdState obd_state() { return state; }
 int obd_speed() { return speed_kmh; }
+float obd_voltage() { return voltage; }
+bool obd_is_idle() { return idle; }
+void obd_wake() { req_wake = true; }
+void obd_set_idle_timeout(uint32_t minutes) { idle_timeout_ms = minutes * 60 * 1000; }
 
 String obd_status_text()
 {