Add watchdog, serial file download, and bootloader-mode flash workflow

Watchdog: 30s ESP32 task watchdog auto-resets on lockup. Fed every
main loop iteration. Disabled during deep sleep (CPU off).

File download: 'D' debug command streams file contents by index over
serial. screenshot.py 'dl <index>' downloads to local file.

Flash workflow no longer needs BOOT+RST:
  screenshot.py z  →  esptool --before no_reset write_flash

Gui.h

@@ -159,9 +159,11 @@ extern volatile uint32_t imu_step_count;
 // Sensor logger toggle — set by .ino after IMULogger.h is included
 typedef bool (*gui_log_toggle_fn_t)();
 static gui_log_toggle_fn_t gui_log_toggle_fn = NULL;
-// SD file listing — set by .ino after SD is available
+// SD file listing and download — set by .ino after SD is available
 typedef void (*gui_list_files_fn_t)();
 static gui_list_files_fn_t gui_list_files_fn = NULL;
+typedef void (*gui_download_file_fn_t)(uint8_t index);
+static gui_download_file_fn_t gui_download_file_fn = NULL;
 static bool gui_imu_logging = false;
 // Forward declarations for IMULogger.h variables (defined later in compilation)
 extern bool imu_logging;
@@ -1003,7 +1005,8 @@ void gui_process_serial_byte(uint8_t b) {
         switch (b) {
             case 'T': gui_cmd_payload_len = 5; break;  // x(2) + y(2) + duration(1)
             case 'N': gui_cmd_payload_len = 2; break;  // col(1) + row(1)
-            default:  gui_cmd_payload_len = 0; break;  // S, M, I, F, L — no payload
+            case 'D': gui_cmd_payload_len = 1; break;  // file index(1)
+            default:  gui_cmd_payload_len = 0; break;  // S, M, I, F, L, X, Z — no payload
         }
         gui_cmd_state++;
         if (gui_cmd_payload_len == 0) {
@@ -1336,6 +1339,16 @@ static void gui_cmd_execute() {
             Serial.flush();
             break;
         }
+        case 'D': {  // Download file by index
+            Serial.write(hdr, 4);
+            if (gui_download_file_fn) {
+                gui_download_file_fn(gui_cmd_payload[0]);
+            } else {
+                Serial.println("{\"error\":\"no_sd\"}");
+            }
+            Serial.flush();
+            break;
+        }
         case 'X': {  // Hard reset
             Serial.write(hdr, 4);
             Serial.println("{\"reset\":true}");

RNode_Firmware.ino

@@ -18,6 +18,7 @@
 #include "Utilities.h"
 
 #if BOARD_MODEL == BOARD_TWATCH_ULT
+  #include "esp_task_wdt.h"
   #include "XL9555.h"
   #include "CO5300.h"
   #include "DRV2605.h"
@@ -117,6 +118,13 @@ void setup() {
   #if MCU_VARIANT == MCU_ESP32
     boot_seq();
 
+    // Hardware watchdog — auto-resets on lockup (30s timeout covers
+    // BHI260AP firmware upload which takes ~10s at boot)
+    #if BOARD_MODEL == BOARD_TWATCH_ULT
+      esp_task_wdt_init(30, true);  // 30s timeout, panic on expire
+      esp_task_wdt_add(NULL);       // subscribe current task (loopTask)
+    #endif
+
     // Init shared SPI bus mutex before any SPI users
     #if BOARD_MODEL == BOARD_TWATCH_ULT
       shared_spi_init();
@@ -2008,6 +2016,7 @@ void work_while_waiting() { loop(); }
 
 void loop() {
   #if BOARD_MODEL == BOARD_TWATCH_ULT
+    esp_task_wdt_reset();  // Feed watchdog
     uint32_t _prof_t0 = micros(), _prof_t1;
   #endif
 
@@ -2258,6 +2267,38 @@ void loop() {
           }
           if (shared_spi_mutex) xSemaphoreGive(shared_spi_mutex);
         };
+        gui_download_file_fn = [](uint8_t index) {
+          if (shared_spi_mutex) xSemaphoreTake(shared_spi_mutex, portMAX_DELAY);
+          SPI.begin(SD_CLK, SD_MISO, SD_MOSI, SD_CS);
+          if (SD.begin(SD_CS, SPI, 4000000, "/sd", 5)) {
+            File root = SD.open("/");
+            File f;
+            uint8_t i = 0;
+            while ((f = root.openNextFile())) {
+              if (i == index) {
+                Serial.printf("{\"name\":\"%s\",\"size\":%lu}\n", f.name(), (unsigned long)f.size());
+                uint8_t buf[512];
+                while (f.available()) {
+                  int n = f.read(buf, sizeof(buf));
+                  Serial.write(buf, n);
+                }
+                f.close();
+                root.close();
+                SD.end();
+                if (shared_spi_mutex) xSemaphoreGive(shared_spi_mutex);
+                return;
+              }
+              f.close();
+              i++;
+            }
+            root.close();
+            SD.end();
+            Serial.printf("{\"error\":\"index %d not found\"}\n", index);
+          } else {
+            Serial.println("{\"error\":\"sd_init_failed\"}");
+          }
+          if (shared_spi_mutex) xSemaphoreGive(shared_spi_mutex);
+        };
         #endif
 
         // Enable step counter (low power, always-on)

scripts/screenshot.py

@@ -241,6 +241,46 @@ def cmd_files(s):
     print(f"Timeout ({len(buf)} bytes)")
 
 
+def cmd_download(s, index, output):
+    """Download file from SD card by index"""
+    s.write(PREFIX + b'D' + bytes([index]))
+    s.flush()
+    buf = b""
+    # Wait for header line with filename and size
+    deadline = time.time() + 10
+    while time.time() < deadline:
+        chunk = s.read(max(1, s.in_waiting or 1))
+        if chunk:
+            buf += chunk
+        magic = PREFIX + b"D"
+        idx = buf.find(magic)
+        if idx >= 0:
+            # Find the JSON header line
+            hdr_start = idx + 4
+            nl = buf.find(b"\n", hdr_start)
+            if nl < 0:
+                continue
+            import json
+            info = json.loads(buf[hdr_start:nl])
+            if "error" in info:
+                print(f"Error: {info['error']}")
+                return
+            fname = info["name"]
+            fsize = info["size"]
+            # Read the file data
+            data = buf[nl + 1:]
+            while len(data) < fsize and time.time() < deadline + 30:
+                chunk = s.read(min(4096, fsize - len(data)))
+                if chunk:
+                    data += chunk
+            outname = output or fname.lstrip("/")
+            with open(outname, "wb") as f:
+                f.write(data[:fsize])
+            print(f"Downloaded {fname} ({fsize} bytes) → {outname}")
+            return
+    print(f"Timeout ({len(buf)} bytes)")
+
+
 def cmd_simple(s, cmd_char, label):
     """Send a command, print response, don't wait long"""
     send_cmd(s, ord(cmd_char))
@@ -309,6 +349,11 @@ def main():
     sub.add_parser("files", aliases=["f"],
                     help="List files on SD card")
 
+    dl = sub.add_parser("download", aliases=["dl"],
+                    help="Download file from SD card by index")
+    dl.add_argument("index", type=int, help="File index from 'files' listing")
+    dl.add_argument("-o", "--output", help="Output filename (default: use SD name)")
+
     sub.add_parser("reset", aliases=["x"],
                     help="Hard reset the device")
 
@@ -342,6 +387,8 @@ def main():
             cmd_log(s)
         elif args.command in ("files", "f"):
             cmd_files(s)
+        elif args.command in ("download", "dl"):
+            cmd_download(s, args.index, args.output)
         elif args.command in ("reset", "x"):
             cmd_simple(s, 'X', "Reset sent")
         elif args.command in ("bootloader", "z"):