chore: run the debug build's serial port at 921600

The debug build logs at info level, and each line holds up the task that
logs it for ~9ms at 115200.  At 921600 that drops to ~1ms.  SERIAL_BAUD
defaults to 115200, so the other envs are unchanged.  The ROM's boot
banner still prints at 115200 and shows as garbage in the monitor.

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

platformio.ini

@@ -79,6 +79,10 @@ build_flags =
 	${dualcore.build_flags}
 	-DCORE_DEBUG_LEVEL=3
 	--param=large-unit-insns=1000
+	-DSERIAL_BAUD=921600
+; The info logs hold up the task that logs for ~1ms a line at 921600, ~9ms at
+; 115200.  The ROM's boot banner stays at 115200, so it shows as garbage.
+monitor_speed = 921600
 
 [env:esp32s3]
 extends = esp32

src/SomfyController.ino

@@ -22,6 +22,10 @@
 
 static const char *TAG = "Main";
 #define LOOP_SLOW_MS 100
+// platformio.ini can set its own, along with a matching monitor_speed.
+#ifndef SERIAL_BAUD
+#define SERIAL_BAUD 115200
+#endif
 // How long an updated firmware has to run, and then have the network up, before
 // it is kept.  A reset before then rolls back to the previous firmware.
 #define OTA_PROVEN_MS 60000
@@ -67,7 +71,7 @@ void listDir(const char *dirname, uint8_t levels) {
 }
 
 void setup() {  
-  Serial.begin(115200);
+  Serial.begin(SERIAL_BAUD);
   ESP_LOGI(TAG, "Startup/Boot....");
   esp_core_dump_summary_t summary;
   if (esp_core_dump_get_summary(&summary) == ESP_OK) {