Migrate all ESP32 envs to Arduino-ESP32 3.x (pioarduino)

The registry espressif32 platform still ships Arduino core 2.0.17, which
lacks ESP32-C6 support. Switch every ESP32 env to the pioarduino fork
(Arduino-ESP32 3.x) via a shared ${common.platform_esp32} variable and
consolidate shared settings into [env].

Rewrite the PWM code for the 3.x LEDC API:
- ledcSetup + ledcAttachPin -> ledcAttach(pin, freq, res)
- ledcDetachPin -> ledcDetach
- ledcWrite now addresses PWM by pin instead of channel index

Adds the esp32-c6-devkitc-1 env; all four ESP32 envs build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -152,7 +152,7 @@ static void attachMeters() {
 
     // Release the channel if it's no longer used or its pin changed.
     if (attachedPin[i] != -1 && attachedPin[i] != pin) {
-      ledcDetachPin(attachedPin[i]);
+      ledcDetach(attachedPin[i]);
       pinMode(attachedPin[i], OUTPUT);
       digitalWrite(attachedPin[i], LOW);  // drive freed pin low so the meter reads zero
       Serial.printf("[PWM] detach ch%d pin%d\n", i, attachedPin[i]);
@@ -160,8 +160,7 @@ static void attachMeters() {
     }
 
     if (pin > 0 && attachedPin[i] != pin) {
-      ledcSetup(i, PWM_FREQ, PWM_RES);
-      ledcAttachPin(pin, i);
+      ledcAttach(pin, PWM_FREQ, PWM_RES);
       attachedPin[i] = pin;
       Serial.printf("[PWM] attach ch%d pin%d\n", i, pin);
     }
@@ -173,7 +172,7 @@ static void applyMeters() {
     if (meters[i].pin <= 0 || meters[i].maxDuty <= 0) continue;
     float pct = meters[i].currentValue / 100.0f * meters[i].maxDuty / 100.0f;
     int duty = constrain((int)(pct * 1023), 0, 1023);
-    ledcWrite(i, duty);
+    ledcWrite(meters[i].pin, duty);
     Serial.printf("[PWM] ch%d duty=%d (cur=%.1f maxD=%.1f)\n", i, duty, meters[i].currentValue, meters[i].maxDuty);
   }
 }
@@ -222,7 +221,7 @@ static void mqttCallback(char* topic, byte* payload, unsigned int len) {
     meters[idx].currentValue = physicalToPct(val, idx);
     if (meters[idx].pin > 0 && meters[idx].maxDuty > 0) {
       float pct = meters[idx].currentValue / 100.0f * meters[idx].maxDuty / 100.0f;
-      ledcWrite(idx, constrain((int)(pct * 1023), 0, 1023));
+      ledcWrite(meters[idx].pin, constrain((int)(pct * 1023), 0, 1023));
     }
     mqttPublishCurrent(idx);
   }
@@ -686,7 +685,7 @@ static void handleSet() {
   if (meters[idx].pin > 0 && meters[idx].maxDuty > 0) {
     float pct = val / 100.0f * meters[idx].maxDuty / 100.0f;
     int duty = constrain((int)(pct * 1023), 0, 1023);
-    ledcWrite(idx, duty);
+    ledcWrite(meters[idx].pin, duty);
   }
   mqttPublishCurrent(idx);
   server.send(200);
@@ -763,8 +762,7 @@ static void factoryReset(bool clearWifi) {
   // Attach meters temporarily for the sweep acknowledgement
   for (int i = 0; i < count && i < 8; i++) {
     if (meters[i].pin > 0) {
-      ledcSetup(i, PWM_FREQ, PWM_RES);
-      ledcAttachPin(meters[i].pin, i);
+      ledcAttach(meters[i].pin, PWM_FREQ, PWM_RES);
     }
   }
 
@@ -776,7 +774,7 @@ static void factoryReset(bool clearWifi) {
       for (int i = 0; i < count && i < 8; i++) {
         if (meters[i].pin > 0 && meters[i].maxDuty > 0) {
           float duty = pct / 100.0f * meters[i].maxDuty / 100.0f;
-          ledcWrite(i, constrain((int)(duty * 1023), 0, 1023));
+          ledcWrite(meters[i].pin, constrain((int)(duty * 1023), 0, 1023));
         }
       }
       delay(50);