Merge pull request 'fix: IWDG reset during gyro recal — refresh at i=0 not i=39 (P0 #42)' (#172) from sl-firmware/gyro-recal-button into main

i%40==39 fired the first IWDG refresh only after 40ms of calibration.
Combined with ~10ms of main loop overhead before entering calibrate(),
total elapsed since last refresh could exceed the 50ms IWDG window.

Change to i%40==0: first refresh fires at i=0 (<1ms after entry),
subsequent refreshes every 40ms — safely within the 50ms window.

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

src/mpu6000.c

@@ -68,8 +68,10 @@ 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();
+        /* Refresh IWDG every 40ms, starting immediately (i=0) — the gap between
+         * safety_refresh() at the top of the main loop and entry here can be
+         * ~10ms, so we must refresh on i=0 to avoid the 50ms IWDG window. */
+        if (i % 40 == 0) safety_refresh();
     }
 
     s_bias_gx = (float)sum_gx / GYRO_CAL_SAMPLES;