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.

test/Makefile

@@ -0,0 +1,52 @@
+CXX ?= g++
+CXXFLAGS ?= -std=c++17 -O0 -g -Wall -Wextra -Wno-unused-parameter
+INCLUDES = -I../components/grub_boot_switch
+
+SRCS = test_disk.cpp \
+       ../components/grub_boot_switch/fat12.cpp \
+       ../components/grub_boot_switch/msc_disk.cpp
+
+BIN = test_disk
+IMG = disk.img
+
+.PHONY: all build run verify clean
+
+all: verify
+
+build: $(BIN)
+
+$(BIN): $(SRCS)
+	$(CXX) $(CXXFLAGS) $(INCLUDES) -o $@ $(SRCS)
+
+# Generate a disk image (target=0 by default; pass T=N to override)
+$(IMG): $(BIN)
+	./$(BIN) $(IMG) $(or $(T),0)
+
+# Run the verifier suite. Requires mtools and `file`.
+verify: $(IMG)
+	@echo "=== file(1) on disk image ==="
+	@file $(IMG)
+	@echo
+	@echo "=== mdir listing ==="
+	@MTOOLS_SKIP_CHECK=1 mdir -i $(IMG) ::
+	@echo
+	@echo "=== switch_position contents ==="
+	@MTOOLS_SKIP_CHECK=1 mtype -i $(IMG) ::switch_position
+	@echo
+	@echo "=== switch_position_grub.cfg contents ==="
+	@MTOOLS_SKIP_CHECK=1 mtype -i $(IMG) ::switch_position_grub.cfg
+	@echo
+	@echo "=== volume serial ==="
+	@MTOOLS_SKIP_CHECK=1 minfo -i $(IMG) :: | grep -i serial
+
+# Cycle through several target values to prove the file content tracks state
+sweep: $(BIN)
+	@for t in 0 1 2 3 9 ; do \
+	    ./$(BIN) sweep_$$t.img $$t >/dev/null ; \
+	    echo "target=$$t:" ; \
+	    MTOOLS_SKIP_CHECK=1 mtype -i sweep_$$t.img ::switch_position_grub.cfg ; \
+	done ; \
+	rm -f sweep_*.img
+
+clean:
+	rm -f $(BIN) $(IMG) sweep_*.img