Extract serial bridge into dedicated classes

This commit refactors the serial bridge functionality out of the `simple_repeater` example and into a more reusable, object-oriented structure.

An `AbstractBridge` interface has been introduced, along with a concrete `SerialBridge` implementation. This encapsulates all the logic for packet framing, checksum calculation, and serial communication, cleaning up the main example file significantly.

The `simple_repeater` example now instantiates and uses the `SerialBridge` class, resulting in better separation of concerns and improved code organization.

src/helpers/SerialBridge.cpp

@@ -0,0 +1,98 @@
+#include "SerialBridge.h"
+#include <HardwareSerial.h>
+
+#ifdef BRIDGE_OVER_SERIAL
+
+#define SERIAL_PKT_MAGIC 0xcafe
+
+struct SerialPacket {
+  uint16_t magic, len, crc;
+  uint8_t payload[MAX_TRANS_UNIT];
+  SerialPacket() : magic(SERIAL_PKT_MAGIC), len(0), crc(0) {}
+};
+
+// Fletcher-16
+// https://en.wikipedia.org/wiki/Fletcher%27s_checksum
+inline static uint16_t fletcher16(const uint8_t *bytes, const size_t len) {
+  uint8_t sum1 = 0, sum2 = 0;
+
+  for (size_t i = 0; i < len; i++) {
+    sum1 = (sum1 + bytes[i]) % 255;
+    sum2 = (sum2 + sum1) % 255;
+  }
+
+  return (sum2 << 8) | sum1;
+};
+
+SerialBridge::SerialBridge(Stream& serial, mesh::PacketManager* mgr) : _serial(&serial), _mgr(mgr) {}
+
+void SerialBridge::begin() {
+#if defined(ESP32)
+  ((HardwareSerial*)_serial)->setPins(BRIDGE_OVER_SERIAL_RX, BRIDGE_OVER_SERIAL_TX);
+#elif defined(RP2040_PLATFORM)
+  ((HardwareSerial*)_serial)->setRX(BRIDGE_OVER_SERIAL_RX);
+  ((HardwareSerial*)_serial)->setTX(BRIDGE_OVER_SERIAL_TX);
+#elif defined(STM32_PLATFORM)
+  ((HardwareSerial*)_serial)->setRx(BRIDGE_OVER_SERIAL_RX);
+  ((HardwareSerial*)_serial)->setTx(BRIDGE_OVER_SERIAL_TX);
+#else
+#error SerialBridge was not tested on the current platform
+#endif
+  ((HardwareSerial*)_serial)->begin(115200);
+}
+
+void SerialBridge::onPacketTransmitted(mesh::Packet* packet) {
+  SerialPacket spkt;
+  spkt.len = packet->writeTo(spkt.payload);
+  spkt.crc = fletcher16(spkt.payload, spkt.len);
+  _serial->write((uint8_t *)&spkt, sizeof(SerialPacket));
+}
+
+void SerialBridge::loop() {
+  while (_serial->available()) {
+    onPacketReceived();
+  }
+}
+
+void SerialBridge::onPacketReceived() {
+  static constexpr uint16_t size = sizeof(SerialPacket) + 1;
+  static uint8_t buffer[size];
+  static uint16_t tail = 0;
+
+  buffer[tail] = (uint8_t)_serial->read();
+  tail = (tail + 1) % size;
+
+  // Check for complete packet by looking back to where the magic number should be
+  const uint16_t head = (tail - sizeof(SerialPacket) + size) % size;
+  if ((buffer[head] | (buffer[(head + 1) % size] << 8)) != SERIAL_PKT_MAGIC) {
+    return;
+  }
+
+  uint8_t bytes[MAX_TRANS_UNIT];
+  const uint16_t len = buffer[(head + 2) % size] | (buffer[(head + 3) % size] << 8);
+
+  if (len == 0 || len > sizeof(bytes)) {
+    return;
+  }
+
+  for (size_t i = 0; i < len; i++) {
+    bytes[i] = buffer[(head + 6 + i) % size];
+  }
+
+  const uint16_t crc = buffer[(head + 4) % size] | (buffer[(head + 5) % size] << 8);
+  const uint16_t f16 = fletcher16(bytes, len);
+
+  if ((f16 != crc)) {
+    return;
+  }
+
+  mesh::Packet *new_pkt = _mgr->allocNew();
+  if (new_pkt == NULL) {
+    return;
+  }
+
+  new_pkt->readFrom(bytes, len);
+  _mgr->queueInbound(new_pkt, 0);
+}
+
+#endif