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/fat12.cpp

@@ -0,0 +1,80 @@
+#include "fat12.h"
+
+#include <string.h>
+
+namespace esphome {
+namespace grub_boot_switch {
+
+static inline uint8_t fat_checksum(const char *name) {
+  uint8_t s = name[0];
+  s = (s << 7) + (s >> 1) + name[1];
+  s = (s << 7) + (s >> 1) + name[2];
+  s = (s << 7) + (s >> 1) + name[3];
+  s = (s << 7) + (s >> 1) + name[4];
+  s = (s << 7) + (s >> 1) + name[5];
+  s = (s << 7) + (s >> 1) + name[6];
+  s = (s << 7) + (s >> 1) + name[7];
+  s = (s << 7) + (s >> 1) + name[8];
+  s = (s << 7) + (s >> 1) + name[9];
+  s = (s << 7) + (s >> 1) + name[10];
+  return s;
+}
+
+static uint8_t write_utf16_chars(const char *name, uint8_t *index, uint8_t length, uint8_t write_count,
+                                 uint8_t *output) {
+  for (uint8_t i = write_count; i != 0; --i) {
+    if (*index <= length) {
+      *output++ = name[*index];
+      *output++ = 0x00;
+      ++(*index);
+    } else {
+      *output++ = 0xFF;
+      *output++ = 0xFF;
+    }
+  }
+  return write_count * 2;
+}
+
+uint8_t fat_write_lfn(const char *name, const FatDirEntry *direntry, uint8_t *output) {
+  constexpr uint8_t kCharsPerEntry = 13;
+
+  const size_t length = strlen(name);
+  const uint8_t num_entries = (length + (kCharsPerEntry - 1)) / kCharsPerEntry;
+  const uint8_t checksum = fat_checksum(reinterpret_cast<const char *>(direntry));
+
+  for (uint8_t ordinal = num_entries; ordinal != 0; --ordinal) {
+    uint8_t index = (ordinal - 1) * kCharsPerEntry;
+
+    *output = ordinal | (kFat_LastLFN * (ordinal == num_entries));
+    ++output;
+
+    output += write_utf16_chars(name, &index, length, 5, output);
+
+    output[0] = kFatAttr_LongFileName;
+    output[1] = 0x00;
+    output[2] = checksum;
+    output += 3;
+
+    output += write_utf16_chars(name, &index, length, 6, output);
+
+    output[0] = 0x00;
+    output[1] = 0x00;
+    output += 2;
+
+    output += write_utf16_chars(name, &index, length, 2, output);
+  }
+
+  return sizeof(FatDirEntry) * num_entries;
+}
+
+uint8_t fat_write_dir(const FatDirEntry *direntry, uint8_t *output) {
+  memcpy(output, direntry, sizeof(FatDirEntry));
+  return sizeof(FatDirEntry);
+}
+
+uint16_t fat_date(uint16_t year, uint8_t month, uint8_t day) {
+  return ((year - 1980) << 9) | (month << 5) | day;
+}
+
+}  // namespace grub_boot_switch
+}  // namespace esphome