feat: gyro recalibration button in web UI (#32)

Add 'G' CDC command that disarms and re-runs gyro bias calibration.
safety_refresh() added to calibration loop (every 40ms) so IWDG
does not trip during the 1s blocking re-cal when watchdog is running.
GYRO CAL button in ui/index.html sends 'G' and shows status feedback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/USB_CDC/src/usbd_cdc_if.c

@@ -6,6 +6,7 @@ volatile uint8_t cdc_streaming = 1;  /* auto-stream */
 static volatile uint8_t cdc_port_open = 0;  /* set when host asserts DTR */
 volatile uint8_t cdc_arm_request = 0;    /* set by A command */
 volatile uint8_t cdc_disarm_request = 0; /* set by D command */
+volatile uint8_t cdc_recal_request = 0;  /* set by G command — gyro recalibration */
 
 /*
  * PID tuning command buffer.
@@ -138,6 +139,7 @@ static int8_t CDC_Receive(uint8_t *buf, uint32_t *len) {
         case 'S': cdc_streaming = !cdc_streaming; break;
         case 'A': cdc_arm_request = 1;            break;
         case 'D': cdc_disarm_request = 1;         break;
+        case 'G': cdc_recal_request = 1;          break; /* gyro recalibration */
         case 'R': request_bootloader();            break; /* never returns */
 
         /*

src/main.c

@@ -23,6 +23,7 @@
 extern volatile uint8_t cdc_streaming;      /* set by S command in CDC RX */
 extern volatile uint8_t cdc_arm_request;    /* set by A command */
 extern volatile uint8_t cdc_disarm_request; /* set by D command */
+extern volatile uint8_t cdc_recal_request;  /* set by G command */
 
 /*
  * Apply a PID tuning command string from the USB terminal.
@@ -207,6 +208,14 @@ int main(void) {
             balance_disarm(&bal);
         }
 
+        /* Gyro recalibration — disarm first, then re-sample bias (~1s blocked) */
+        if (cdc_recal_request) {
+            cdc_recal_request = 0;
+            safety_arm_cancel();
+            balance_disarm(&bal);
+            if (imu_ret == 0) mpu6000_calibrate();
+        }
+
         /* Handle PID tuning commands from USB (P/I/D/T/M/?) */
         if (cdc_cmd_ready) {
             cdc_cmd_ready = 0;

src/mpu6000.c

@@ -12,6 +12,7 @@
 #include "mpu6000.h"
 #include "icm42688.h"
 #include "config.h"
+#include "safety.h"
 #include "stm32f7xx_hal.h"
 #include <math.h>
 
@@ -67,6 +68,8 @@ void mpu6000_calibrate(void) {
         sum_gy += raw.gy;
         sum_gz += raw.gz;
         HAL_Delay(1);
+        /* Refresh IWDG every 40ms — safe during re-cal with watchdog running */
+        if (i % 40 == 39) safety_refresh();
     }
 
     s_bias_gx = (float)sum_gx / GYRO_CAL_SAMPLES;

src/safety.c

@@ -42,7 +42,7 @@ void safety_init(void) {
 }
 
 void safety_refresh(void) {
-    HAL_IWDG_Refresh(&hiwdg);
+    if (hiwdg.Instance) HAL_IWDG_Refresh(&hiwdg);
 }
 
 bool safety_rc_alive(uint32_t now) {

ui/index.html

@@ -26,6 +26,8 @@
   #arm-btn.armed { background: #ff2222; }
   #dfu-btn { background: #555; display: none; }
   #dfu-btn:hover { background: #777; }
+  #gyrocal-btn { background: #1a4a6a; display: none; }
+  #gyrocal-btn:hover { background: #2a6a9a; }
   #status { margin-top: 8px; font-size: 12px; color: #666; }
   #state-badge { 
     display: inline-block; padding: 2px 8px; border-radius: 3px; 
@@ -62,6 +64,7 @@
     <button class="btn" id="arm-btn" onclick="toggleArm()">ARM</button>
     <button class="btn" id="dfu-btn" onclick="enterDFU()">DFU</button>
     <button class="btn" id="yaw-btn" onclick="resetYaw()" style="background:#335533;display:none">YAW RESET</button>
+    <button class="btn" id="gyrocal-btn" onclick="gyroRecal()">GYRO CAL</button>
   </div>
   <div id="status">WebSerial ready</div>
 </div>
@@ -315,6 +318,7 @@ window.toggleSerial = async function() {
     document.getElementById('arm-btn').style.display = 'none';
     document.getElementById('dfu-btn').style.display = 'none';
     document.getElementById('yaw-btn').style.display = 'none';
+    document.getElementById('gyrocal-btn').style.display = 'none';
     document.getElementById('status').textContent = 'Disconnected';
     return;
   }
@@ -329,6 +333,7 @@ window.toggleSerial = async function() {
     document.getElementById('arm-btn').style.display = 'inline-block';
     document.getElementById('dfu-btn').style.display = 'inline-block';
     document.getElementById('yaw-btn').style.display = 'inline-block';
+    document.getElementById('gyrocal-btn').style.display = 'inline-block';
     document.getElementById('status').textContent = 'Connected — streaming';
 
     readLoop();
@@ -353,6 +358,11 @@ window.enterDFU = async function() {
   document.getElementById('status').textContent = 'Rebooting to DFU...';
 };
 
+window.gyroRecal = async function() {
+  await sendCmd('G');
+  document.getElementById('status').textContent = 'Calibrating gyro — hold robot still for 1s...';
+};
+
 async function readLoop() {
   const decoder = new TextDecoderStream();
   port.readable.pipeTo(decoder.writable);