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/__init__.py

@@ -0,0 +1,102 @@
+"""ESPHome external_component: GRUB boot switch over USB MSC.
+
+Presents a synthetic 64 KB FAT12 disk to the attached host containing two
+virtual files:
+
+  switch_position           - one ASCII digit (0-9) reflecting the current
+                              boot target.
+  switch_position_grub.cfg  - GRUB snippet `set os_hw_switch=N\\n` consumed
+                              by the user's /etc/grub.d/01_bootswitch hook.
+
+The volume serial defaults to 0x55AA6922 to stay drop-in compatible with
+the upstream stecman/hw-boot-selection firmware.
+"""
+
+from esphome import automation
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.const import CONF_ID
+from esphome.components import esp32
+
+DEPENDENCIES = ["esp32"]
+CODEOWNERS = ["@adamksmith"]
+MULTI_CONF = False
+
+grub_boot_switch_ns = cg.esphome_ns.namespace("grub_boot_switch")
+GrubBootSwitch = grub_boot_switch_ns.class_("GrubBootSwitch", cg.Component)
+
+CONF_VOLUME_SERIAL = "volume_serial"
+CONF_VOLUME_LABEL = "volume_label"
+CONF_USB_VID = "usb_vid"
+CONF_USB_PID = "usb_pid"
+CONF_USB_MANUFACTURER = "usb_manufacturer"
+CONF_USB_PRODUCT = "usb_product"
+CONF_INITIAL_TARGET = "initial_target"
+CONF_TARGETS = "targets"
+CONF_GRUB_BOOT_SWITCH_ID = "grub_boot_switch_id"
+
+
+def _validate_label(value):
+    value = cv.string_strict(value)
+    if len(value) > 11:
+        raise cv.Invalid("volume_label must be 11 characters or fewer")
+    return value
+
+
+# targets: maps a digit (0-9) the firmware writes into switch_position to a
+# human-readable label.  GRUB scripts react to the digit; the labels are only
+# used by the select sub-platform exposed to Home Assistant.
+TARGETS_SCHEMA = cv.Schema({cv.int_range(min=0, max=9): cv.string_strict})
+
+
+CONFIG_SCHEMA = cv.All(
+    cv.Schema(
+        {
+            cv.GenerateID(): cv.declare_id(GrubBootSwitch),
+            cv.Optional(CONF_VOLUME_SERIAL, default=0x55AA6922): cv.hex_uint32_t,
+            cv.Optional(CONF_VOLUME_LABEL, default="SWITCH"): _validate_label,
+            cv.Optional(CONF_USB_VID, default=0x26BA): cv.hex_uint16_t,
+            cv.Optional(CONF_USB_PID, default=0x8003): cv.hex_uint16_t,
+            cv.Optional(CONF_USB_MANUFACTURER, default="Stecman"): cv.string,
+            cv.Optional(CONF_USB_PRODUCT, default="Boot Switch"): cv.string,
+            cv.Optional(CONF_INITIAL_TARGET, default=0): cv.int_range(min=0, max=9),
+            cv.Optional(CONF_TARGETS, default={}): TARGETS_SCHEMA,
+        }
+    ).extend(cv.COMPONENT_SCHEMA),
+    cv.only_with_esp_idf,
+    cv.only_on_esp32,
+)
+
+
+async def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID])
+    await cg.register_component(var, config)
+
+    cg.add(var.set_volume_serial(config[CONF_VOLUME_SERIAL]))
+    cg.add(var.set_volume_label(config[CONF_VOLUME_LABEL]))
+    cg.add(var.set_usb_ids(config[CONF_USB_VID], config[CONF_USB_PID]))
+    cg.add(var.set_usb_strings(config[CONF_USB_MANUFACTURER], config[CONF_USB_PRODUCT]))
+    cg.add(var.set_initial_target(config[CONF_INITIAL_TARGET]))
+
+    for digit, label in sorted(config[CONF_TARGETS].items()):
+        cg.add(var.add_target(digit, label))
+
+    # esp_tinyusb is the supported MSC stack on ESP32-S3 USB-OTG.
+    esp32.add_idf_component(
+        name="espressif/esp_tinyusb",
+        ref="~1.4.0",
+    )
+
+    # Move IDF console off USB-Serial-JTAG so TinyUSB can claim the peripheral.
+    esp32.add_idf_sdkconfig_option("CONFIG_ESP_CONSOLE_UART_DEFAULT", True)
+    esp32.add_idf_sdkconfig_option("CONFIG_ESP_CONSOLE_USB_SERIAL_JTAG", False)
+
+    # esp_tinyusb's tusb_msc_storage.c provides default tud_msc_* callbacks as
+    # strong symbols. We provide our own callback-driven implementations, so
+    # we tell the linker to allow multiple definitions and rely on link order
+    # (main component first → our symbols win) to pick ours.
+    esp32.add_idf_sdkconfig_option("CONFIG_TINYUSB_MSC_ENABLED", True)
+    esp32.add_idf_sdkconfig_option("CONFIG_TINYUSB_CDC_ENABLED", False)
+
+    cg.add_build_flag("-DCFG_TUSB_MCU=OPT_MCU_ESP32S3")
+    cg.add_build_flag("-Wl,--allow-multiple-definition")