feat: add Bluetooth LE mode, radio power budget and NINA firmware update path

arduino/strix_nina_passthrough/strix_nina_passthrough.ino

@@ -0,0 +1,57 @@
+// STRIX NINA passthrough: connects the USB serial port to the NINA-W102 WiFi module's UART so
+// esptool can flash its firmware. The STRIX FPGA image passes the module's UART (D14 -> RX,
+// D13 <- TX), GPIO0 (D6) and RESETN (D7) through to the SAMD21 without inversion; asserted USB
+// RTS holds the module in reset and asserted DTR pulls GPIO0 low, as esptool's reset sequence
+// expects. Based on Arduino's SerialNINAPassthrough.
+
+#include <StrixFPGA.h>
+
+using namespace strix;
+
+const int NINA_GPIO0_PIN = 6;
+const int NINA_RESETN_PIN = 7;
+
+StrixFPGA fpga;
+unsigned long baud = 115200;
+int rts = -1;
+int dtr = -1;
+
+void setup() {
+  Serial.begin(baud);
+  fpga.begin();
+  fpga.write(REG_GEN_ENABLE, 0);  // reduce load while the module runs
+  fpga.write(REG_NINA_CTRL, 1);   // let the module out of reset
+
+  Serial1.begin(baud);
+  pinMode(NINA_GPIO0_PIN, OUTPUT);
+  pinMode(NINA_RESETN_PIN, OUTPUT);
+}
+
+void loop() {
+  if (rts != Serial.rts()) {
+    digitalWrite(NINA_RESETN_PIN, Serial.rts() ? LOW : HIGH);
+    rts = Serial.rts();
+  }
+  if (dtr != Serial.dtr()) {
+    digitalWrite(NINA_GPIO0_PIN, Serial.dtr() ? LOW : HIGH);  // DTR asserted: download mode
+    dtr = Serial.dtr();
+  }
+
+  // Forward in chunks: one USB write per byte is too slow at esptool's rates and lets the UART's
+  // receive buffer overflow.
+  static uint8_t buffer[256];
+  size_t count = 0;
+  while (Serial1.available() && count < sizeof(buffer)) buffer[count++] = Serial1.read();
+  if (count) Serial.write(buffer, count);
+  count = 0;
+  while (Serial.available() && count < sizeof(buffer)) buffer[count++] = Serial.read();
+  if (count) Serial1.write(buffer, count);
+
+  // Follow baud rate changes requested by esptool.
+  if (Serial.baud() != baud) {
+    rts = -1;
+    dtr = -1;
+    baud = Serial.baud();
+    Serial1.begin(baud);
+  }
+}