Harden Si4684 RSTB config and document power sequencing.

Enable GPIO_PULLDOWN_ENABLE during gpio_config so RSTB cannot glitch high before gpio_set_level(0), and add manual validation notes explaining why the internal pull-down complements the external one.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/components/drivers/si4684/include/si4684/Si4684Driver.hpp

@@ -96,7 +96,14 @@ public:
     Si4684Driver& operator=(const Si4684Driver&) = delete;
 
     /**
-     * @brief    boot — cold-start: load patch, image, and configure I/O.
+     * @brief    boot — cold-start: pulse RSTB#, load patch/image, configure I/O.
+     *
+     * @details  Pulses RSTB# (active-low reset): gpio_config enables the ESP32
+     *           internal pull-down while switching GPIO38 to output so the net
+     *           cannot glitch high before gpio_set_level(0). That guards the
+     *           Si4684 datasheet rule that RSTB must stay low until supplies are
+     *           stable; the board's external pull-down already holds RSTB during
+     *           rail ramp before app_main runs.
      *
      * @dname    boot
      * @param    band  DAB or FM application to load.

Software/components/drivers/si4684/src/Si4684Driver.cpp

@@ -389,6 +389,7 @@ std::expected<void, Si4684Error> Si4684Driver::boot(Si4684Band band)
     gpio_config_t rstCfg = {};
     rstCfg.pin_bit_mask = 1ULL << pins_.rstbGpio;
     rstCfg.mode = GPIO_MODE_OUTPUT;
+    rstCfg.pull_down_en = GPIO_PULLDOWN_ENABLE;
     if (gpio_config(&rstCfg) != ESP_OK) {
         return std::unexpected(Si4684Error::ResetFailed);
     }