Keep ESP32 OTA reachable whatever the firmware does

- Rollback: override verifyRollbackLater() so a new OTA image stays
  pending until it ran 60 s with WiFi; any earlier restart rolls back
  (the Arduino core marked every image valid at boot before)
- Task watchdog (30 s) on loop(), paused during OTA uploads
- Crash counter in RTC memory: OTA-only window after a crash, 10 min
  safe mode (WiFi + OTA only) after 3 crashes in a row
- Restart when WiFi stays down for 10 min
- Home Assistant sensor 'FlipDot Display Firmware' with version/state
- logic/boot_policy with tests (60 tests total)

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

src/esp32/net/home_assistant.cpp

@@ -2,6 +2,7 @@
 
 #include <ArduinoHA.h>
 #include <WiFi.h>
+#include <string.h>
 
 #include "config.h"
 #include "credentials.h"
@@ -13,6 +14,10 @@ static HAMqtt mqtt(client, device);
 // Unique ids: do not change, Home Assistant's entities are linked to them
 static HASwitch clockSwitch("FlipDotDisplayClock");
 static HASwitch powerSwitch("FlipDotDisplayPower");
+static HASensor firmwareSensor("FlipDotDisplayFirmware");
+
+static char firmware_status[64] = "";
+static bool firmware_status_sent = false;
 
 static DisplayController *displayController = nullptr;
 
@@ -46,10 +51,25 @@ void homeAssistantBegin(DisplayController &controller)
 	powerSwitch.setCurrentState(controller.powerEnabled());
 	powerSwitch.onCommand(onPowerCommand);
 
+	firmwareSensor.setName("FlipDot Display Firmware");
+	firmwareSensor.setIcon("mdi:chip");
+	// The sensor doesn't resend its value on reconnect by itself
+	mqtt.onConnected([]() { firmware_status_sent = false; });
+
 	mqtt.begin(BROKER_ADDR, MQTT_USER, MQTT_PASSWORD);
 }
 
 void homeAssistantLoop()
 {
 	mqtt.loop();
+	if (!firmware_status_sent && firmware_status[0] && mqtt.isConnected())
+		firmware_status_sent = firmwareSensor.setValue(firmware_status);
+}
+
+void homeAssistantSetFirmwareStatus(const char *status)
+{
+	if (strcmp(status, firmware_status) == 0)
+		return;
+	strncpy(firmware_status, status, sizeof(firmware_status) - 1);
+	firmware_status_sent = false;
 }