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/usb_descriptors.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace esphome {
+namespace grub_boot_switch {
+
+// Built at runtime from configuration values so VID/PID/strings can be tuned
+// from YAML.  Lifetime: descriptors live forever once built (single device).
+struct UsbDescriptorBundle {
+  // tusb_desc_device_t struct (18 bytes), kept opaque here to avoid leaking
+  // tusb headers into the rest of the component.
+  uint8_t device[18];
+
+  // Configuration descriptor: 9 (config) + 9 (interface) + 2*7 (endpoints) = 32 bytes.
+  uint8_t config[32];
+
+  // String descriptors: index 0 = lang (English), 1 = manufacturer,
+  // 2 = product, 3 = serial.  ASCII C-strings, null terminated.
+  const char *strings[4];
+  char serial_buf[24];  // backing store for the auto-generated serial.
+};
+
+// Populate the bundle from runtime values.  The serial_buf is filled with the
+// chip's MAC address ("E072A1D54520") if `serial_override` is null/empty.
+void usb_descriptors_build(UsbDescriptorBundle *bundle, uint16_t vid, uint16_t pid,
+                           const char *manufacturer, const char *product,
+                           const char *serial_override);
+
+}  // namespace grub_boot_switch
+}  // namespace esphome