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/grub_boot_switch.cpp

@@ -0,0 +1,116 @@
+#include "grub_boot_switch.h"
+#include "usb_descriptors.h"
+
+#include "tinyusb.h"
+
+namespace esphome {
+namespace grub_boot_switch {
+
+static const char *const TAG = "grub_boot_switch";
+
+extern GrubBootSwitch *g_active_component;  // defined in tusb_glue.cpp
+
+static UsbDescriptorBundle s_usb_descriptors;
+
+std::string GrubBootSwitch::label_for_digit(uint8_t digit) const {
+  for (const auto &t : targets_) {
+    if (t.first == digit)
+      return t.second;
+  }
+  return std::string();
+}
+
+bool GrubBootSwitch::digit_for_label(const std::string &label, uint8_t *out) const {
+  for (const auto &t : targets_) {
+    if (t.second == label) {
+      if (out)
+        *out = t.first;
+      return true;
+    }
+  }
+  return false;
+}
+
+void GrubBootSwitch::set_target(uint8_t t) {
+  if (current_target_.load() == t)
+    return;
+  current_target_.store(t);
+  pref_.save(&t);
+  for (auto &cb : callbacks_)
+    cb(t);
+}
+
+void GrubBootSwitch::install_virtual_files_() {
+  // File 1: bare ASCII digit, one byte. Mirrors readSwitch() in the reference.
+  disk_.add_file("switch_position", "SWITCH~1", "   ", 1,
+                 [this](uint8_t *out) { out[0] = '0' + (current_target_.load() % 10); });
+
+  // File 2: GRUB snippet. Length and final-byte position match the reference's
+  // grubConfigStr ("set os_hw_switch=N\n") so existing /etc/grub.d hooks parse
+  // it identically.
+  static const char kPrefix[] = "set os_hw_switch=";
+  static const size_t kPrefixLen = sizeof(kPrefix) - 1;
+  static const size_t kFileLen = kPrefixLen + 2;  // digit + newline
+  disk_.add_file("switch_position_grub.cfg", "SWITCH~1", "CFG", kFileLen,
+                 [this](uint8_t *out) {
+                   memcpy(out, kPrefix, kPrefixLen);
+                   out[kPrefixLen] = '0' + (current_target_.load() % 10);
+                   out[kPrefixLen + 1] = '\n';
+                 });
+}
+
+void GrubBootSwitch::start_usb_() {
+  g_active_component = this;
+
+  usb_descriptors_build(&s_usb_descriptors, usb_vid_, usb_pid_,
+                        usb_manufacturer_.c_str(), usb_product_.c_str(), nullptr);
+
+  tinyusb_config_t cfg = {};
+  cfg.device_descriptor =
+      reinterpret_cast<const tusb_desc_device_t *>(s_usb_descriptors.device);
+  cfg.string_descriptor = s_usb_descriptors.strings;
+  cfg.string_descriptor_count =
+      sizeof(s_usb_descriptors.strings) / sizeof(s_usb_descriptors.strings[0]);
+  cfg.external_phy = false;
+  cfg.configuration_descriptor = s_usb_descriptors.config;
+  cfg.self_powered = false;
+
+  esp_err_t err = tinyusb_driver_install(&cfg);
+  if (err != ESP_OK) {
+    ESP_LOGE(TAG, "tinyusb_driver_install failed: %d", err);
+    this->mark_failed();
+    return;
+  }
+  ESP_LOGI(TAG, "TinyUSB MSC installed (VID=%04X PID=%04X serial=%s)", usb_vid_,
+           usb_pid_, s_usb_descriptors.serial_buf);
+}
+
+void GrubBootSwitch::setup() {
+  ESP_LOGCONFIG(TAG, "Setting up GRUB boot switch");
+
+  // NVS-backed persistence: hash is "grub_boot_switch::target" + a fixed key
+  // so updates stay tied to this component instance.
+  pref_ = global_preferences->make_preference<uint8_t>(0x47425354U /* "GBST" */, true);
+  uint8_t saved = current_target_.load();
+  if (pref_.load(&saved)) {
+    current_target_.store(saved);
+    ESP_LOGCONFIG(TAG, "Restored target from NVS: %u", saved);
+  }
+
+  install_virtual_files_();
+  start_usb_();
+}
+
+void GrubBootSwitch::dump_config() {
+  ESP_LOGCONFIG(TAG, "GRUB boot switch:");
+  ESP_LOGCONFIG(TAG, "  USB VID/PID: %04X:%04X", usb_vid_, usb_pid_);
+  ESP_LOGCONFIG(TAG, "  Current target: %u", current_target_.load());
+  if (!targets_.empty()) {
+    ESP_LOGCONFIG(TAG, "  Targets:");
+    for (const auto &t : targets_)
+      ESP_LOGCONFIG(TAG, "    %u: %s", t.first, t.second.c_str());
+  }
+}
+
+}  // namespace grub_boot_switch
+}  // namespace esphome