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.

example.yaml

@@ -0,0 +1,36 @@
+esphome:
+  name: grub-boot-switch
+  friendly_name: GRUB boot switch
+
+esp32:
+  board: esp32-s3-devkitc-1
+  framework:
+    type: esp-idf
+
+logger:
+api:
+ota:
+  - platform: esphome
+
+wifi:
+  ssid: !secret wifi_ssid
+  password: !secret wifi_password
+
+external_components:
+  - source:
+      type: local
+      path: components
+    components: [grub_boot_switch]
+
+grub_boot_switch:
+  id: bootsw
+  # volume_serial: 0x55AA6922   # default — matches upstream GRUB hook
+  # volume_label: SWITCH        # default
+  initial_target: 0
+
+# Phase 4 will add the select platform here:
+#
+# select:
+#   - platform: grub_boot_switch
+#     name: "Boot Target"
+#     options: [Linux, Windows, Recovery]