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.

.gitignore

@@ -0,0 +1,26 @@
+# ESPHome build cache
+/.esphome/
+
+# Device YAML + secrets live in the user's HA config repo, not here
+/ctn1-usco-wks-boot-selector.yaml
+/secrets.yaml
+
+# Upstream reference firmware — separate repo, clone manually for context:
+#   git clone https://github.com/stecman/hw-boot-selection.git reference
+/reference/
+
+# Host-side test outputs
+/test/test_disk
+/test/disk.img
+/test/sweep_*.img
+/test/*.img
+
+# Python / editor / OS
+__pycache__/
+*.pyc
+*.pyo
+.DS_Store
+*.swp
+.vscode/
+.idea/
+.claude/