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.h

@@ -0,0 +1,82 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/core/log.h"
+#include "esphome/core/preferences.h"
+
+#include "msc_disk.h"
+
+#include <atomic>
+#include <functional>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace esphome {
+namespace grub_boot_switch {
+
+class GrubBootSwitch : public Component {
+ public:
+  void setup() override;
+  void dump_config() override;
+  float get_setup_priority() const override { return setup_priority::HARDWARE; }
+
+  void set_volume_serial(uint32_t s) { disk_.set_volume_serial(s); }
+  void set_volume_label(const std::string &l) { disk_.set_volume_label(l.c_str()); }
+  void set_usb_ids(uint16_t vid, uint16_t pid) {
+    usb_vid_ = vid;
+    usb_pid_ = pid;
+  }
+  void set_usb_strings(const std::string &manuf, const std::string &product) {
+    usb_manufacturer_ = manuf;
+    usb_product_ = product;
+  }
+  void set_initial_target(uint8_t t) { current_target_ = t; }
+
+  // Target table: digit (0-9) → human-readable label.  Populated from YAML
+  // before setup() runs.  Order matters only for select-platform option lists.
+  void add_target(uint8_t digit, const std::string &label) {
+    targets_.emplace_back(digit, label);
+  }
+  const std::vector<std::pair<uint8_t, std::string>> &get_targets() const { return targets_; }
+
+  // Look up the label for a digit.  Returns empty string if no match.
+  std::string label_for_digit(uint8_t digit) const;
+  // Look up the digit for a label.  Returns false on miss; *out unchanged.
+  bool digit_for_label(const std::string &label, uint8_t *out) const;
+
+  // Public API used by automations + sub-components.  set_target() persists
+  // to NVS and notifies registered callbacks.
+  uint8_t get_target() const { return current_target_.load(); }
+  void set_target(uint8_t t);
+
+  // Registered by the select sub-platform so it can publish state changes
+  // when set_target() is called from REST/automation/etc.
+  void add_on_target_change(std::function<void(uint8_t)> &&cb) {
+    callbacks_.emplace_back(std::move(cb));
+  }
+
+  // Called by the TinyUSB MSC glue layer.
+  int read_lba(uint32_t lba, uint8_t *out) { return disk_.read_lba(lba, out); }
+  uint32_t sector_count() const { return disk_.sector_count(); }
+  uint32_t sector_size() const { return disk_.sector_size(); }
+
+ protected:
+  void install_virtual_files_();
+  void start_usb_();
+
+  MscDisk disk_;
+  std::atomic<uint8_t> current_target_{0};
+  std::vector<std::pair<uint8_t, std::string>> targets_;
+  std::vector<std::function<void(uint8_t)>> callbacks_;
+
+  ESPPreferenceObject pref_;
+
+  uint16_t usb_vid_ = 0x26BA;
+  uint16_t usb_pid_ = 0x8003;
+  std::string usb_manufacturer_;
+  std::string usb_product_;
+};
+
+}  // namespace grub_boot_switch
+}  // namespace esphome