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/select/__init__.py

@@ -0,0 +1,51 @@
+"""Select sub-platform for grub_boot_switch.
+
+Exposes the parent's targets table as a Home Assistant select entity.  The
+options list is derived at code-gen time from the parent's `targets:` map, so
+adding/removing entries there automatically updates the dropdown.
+"""
+
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import select
+from esphome.const import CONF_ID
+
+from .. import (
+    CONF_GRUB_BOOT_SWITCH_ID,
+    CONF_TARGETS,
+    GrubBootSwitch,
+    grub_boot_switch_ns,
+)
+
+DEPENDENCIES = ["grub_boot_switch"]
+
+BootTargetSelect = grub_boot_switch_ns.class_(
+    "BootTargetSelect", select.Select, cg.Component
+)
+
+CONFIG_SCHEMA = select.select_schema(BootTargetSelect).extend(
+    {
+        cv.GenerateID(CONF_GRUB_BOOT_SWITCH_ID): cv.use_id(GrubBootSwitch),
+    }
+)
+
+
+async def to_code(config):
+    parent = await cg.get_variable(config[CONF_GRUB_BOOT_SWITCH_ID])
+
+    # Reach back into the *full* validated config so we can pull the parent's
+    # targets map.  CORE.config holds the post-validation tree.
+    from esphome.core import CORE
+
+    parent_cfg = CORE.config.get("grub_boot_switch", {})
+    targets = parent_cfg.get(CONF_TARGETS, {})
+    if not targets:
+        raise cv.Invalid(
+            "select platform requires the grub_boot_switch component to define `targets:`"
+        )
+
+    options = [label for _digit, label in sorted(targets.items())]
+
+    var = await select.new_select(config, options=options)
+    await cg.register_component(var, config)
+    cg.add(var.set_parent(parent))

components/grub_boot_switch/select/boot_target_select.cpp

@@ -0,0 +1,44 @@
+#include "boot_target_select.h"
+
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace grub_boot_switch {
+
+static const char *const TAG = "grub_boot_switch.select";
+
+void BootTargetSelect::setup() {
+  // Publish the current state from the parent so HA reflects whatever was
+  // restored from NVS / set_initial_target.
+  if (parent_ != nullptr) {
+    auto label = parent_->label_for_digit(parent_->get_target());
+    if (!label.empty())
+      this->publish_state(label);
+
+    // Mirror future external set_target() calls (REST/automation/MQTT) into
+    // the select's reported state.
+    parent_->add_on_target_change([this](uint8_t digit) {
+      auto l = this->parent_->label_for_digit(digit);
+      if (!l.empty())
+        this->publish_state(l);
+    });
+  }
+}
+
+void BootTargetSelect::control(const std::string &value) {
+  if (parent_ == nullptr)
+    return;
+  uint8_t digit = 0;
+  if (!parent_->digit_for_label(value, &digit)) {
+    ESP_LOGW(TAG, "control: unknown option '%s'", value.c_str());
+    return;
+  }
+  parent_->set_target(digit);
+  // The parent will fire add_on_target_change → publish_state, but if the
+  // target was already at this digit no callback fires; publish anyway so
+  // HA's optimistic state stays in sync.
+  this->publish_state(value);
+}
+
+}  // namespace grub_boot_switch
+}  // namespace esphome

components/grub_boot_switch/select/boot_target_select.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/select/select.h"
+
+#include "../grub_boot_switch.h"
+
+namespace esphome {
+namespace grub_boot_switch {
+
+class BootTargetSelect : public select::Select, public Component {
+ public:
+  void set_parent(GrubBootSwitch *parent) { parent_ = parent; }
+
+  void setup() override;
+
+ protected:
+  void control(const std::string &value) override;
+
+  GrubBootSwitch *parent_{nullptr};
+};
+
+}  // namespace grub_boot_switch
+}  // namespace esphome