Initial port of stecman/hw-boot-selection to ESP32-S3

ESPHome external_component that presents a synthetic 64 KB FAT12 disk
over the ESP32-S3's native USB-OTG, containing a one-byte switch_position
file and a switch_position_grub.cfg snippet.  The disk's volume serial
is fixed at 0x55AA6922 to stay drop-in compatible with the upstream
/etc/grub.d hook.

Components:
- fat12 / msc_disk: portable FAT12 sector synthesiser ported from
  reference/src/fat.c and the boot-sector / FAT / dir-entry generation
  in reference/src/usb.c.
- usb_descriptors / tusb_glue: TinyUSB MSC class with our own callbacks
  delegating to MscDisk::read_lba (linker --allow-multiple-definition
  picks ours over esp_tinyusb's storage-backed defaults).
- grub_boot_switch: top-level Component, NVS persistence on target
  changes, exposes targets table to sub-platforms.
- select: HA-facing select platform reading parent's targets map.

Validated end-to-end on an ESP32-S3-DevKitC-1: USB enumerates as
26ba:8003, /dev/sdb mounts FAT12, mtype reads back the live target,
and POST /select/boot_target/set?option=Windows updates both files
on the next host read.

components/grub_boot_switch/select/boot_target_select.cpp

@@ -0,0 +1,44 @@
+#include "boot_target_select.h"
+
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace grub_boot_switch {
+
+static const char *const TAG = "grub_boot_switch.select";
+
+void BootTargetSelect::setup() {
+  // Publish the current state from the parent so HA reflects whatever was
+  // restored from NVS / set_initial_target.
+  if (parent_ != nullptr) {
+    auto label = parent_->label_for_digit(parent_->get_target());
+    if (!label.empty())
+      this->publish_state(label);
+
+    // Mirror future external set_target() calls (REST/automation/MQTT) into
+    // the select's reported state.
+    parent_->add_on_target_change([this](uint8_t digit) {
+      auto l = this->parent_->label_for_digit(digit);
+      if (!l.empty())
+        this->publish_state(l);
+    });
+  }
+}
+
+void BootTargetSelect::control(const std::string &value) {
+  if (parent_ == nullptr)
+    return;
+  uint8_t digit = 0;
+  if (!parent_->digit_for_label(value, &digit)) {
+    ESP_LOGW(TAG, "control: unknown option '%s'", value.c_str());
+    return;
+  }
+  parent_->set_target(digit);
+  // The parent will fire add_on_target_change → publish_state, but if the
+  // target was already at this digit no callback fires; publish anyway so
+  // HA's optimistic state stays in sync.
+  this->publish_state(value);
+}
+
+}  // namespace grub_boot_switch
+}  // namespace esphome