v2.12.5-aircast.1 — native Aircast USB mode

Based on upstream EdgeTX v2.12.5.

Adds a new USB-VCP serial mode, Aircast (SYS → Hardware → Serial Port →
USB-VCP → Aircast), that streams the model's mixed channel outputs over
USB in the same text protocol the Aircast app's aircast-usb.lua script
already reads: $ch1,ch2,...,ch24,\r\n. No script to install on the SD
card, no Tools screen to keep open — it streams in the background the
moment the port is set to Aircast, from any menu.

Verified on a real RadioMaster TX16S: flashes, boots, the Hardware screen
offers Aircast alongside LUA/CLI/Telem Mirror, and selecting it streams
sticks to the app exactly like the script did.

Flashing

Flash tx16s-79692ae0dd.bin via buddy.edgetx.org → Local file tab (not
the version catalog). Setup guide:
https://github.com/pavliha/aircast-sitl/blob/master/../../qgroundcontrol/tools/edgetx/README.md

Known gap

EdgeTX Companion doesn't know the "Aircast" mode name yet, so opening this
radio's settings in Companion and saving will silently reset Serial Port →
VCP back to OFF. Configure it on-device instead.