…p pre-volume

# Conflicts:
#	code/src/PhoenixSketch/Loop.cpp
#	code/src/PhoenixSketch/SDT.h

Upstream renamed BUFFER_SIZE to USB_BUFFER_SIZE in SDT.h to avoid a
namespace conflict with another library, and updated DSP.cpp's non-FT8
code accordingly. This left three references in the FT8 additions
(PlayUsbBufferPreVol's test tone generator and ReadMicrophoneBuffer's
FT8 sample path) still using the now-undefined BUFFER_SIZE macro.

Ft8UsbBridge_DrainToUSB() was the only project code calling a
core_mods-only function (usb_audio_push_block) -- confirmed by searching
src/ for it and the other three core_mods additions
(usb_audio_pop_receive_block, usb_audio_tx_has_room,
usb_audio_manual_transmit), none of which are otherwise referenced.

core_mods/usb_audio.h now defines T41_CORE_HAS_USB_AUDIO_PUSH_BLOCK next to
the push_block declaration, so project code can detect at compile time
whether the patched core is installed. Anyone re-copying this header into
a Teensyduino install must bring this #define along -- applied here to
both the repo copy and the installed copy at
AppData/Local/Arduino15/packages/teensy/hardware/avr/1.62.0/cores/teensy4/.

When the sentinel is absent, Ft8UsbBridge_DrainToUSB() drops the buffered
RX samples instead of calling the missing function, with a #warning at the
top of the file explaining why. FT8 TX (PC -> radio) is unaffected since it
never depended on the core patch.

compile.yml gains a second, independent job (compile-usb-audio) that
builds with usb=serialmidiaudio against CI's stock-installed core, so the
FT8 code path actually gets compiled and the new gate gets exercised
instead of silently skipping (the existing usb=serial2 job never defines
USB_AUDIO/USB_MIDI_AUDIO_SERIAL, so it excludes this code entirely and
proves nothing about it).

… just T41_USB_AUDIO

usb_audio.h wraps AudioInputUSB/AudioOutputUSB themselves in
#ifdef AUDIO_INTERFACE, which boards.txt only defines for USB Type =
Audio / Serial+MIDI+Audio. T41_USB_AUDIO (Config.h) is independent of that
board menu selection, so a usb=serial2 build previously still tried to
declare AudioInputUSB g_usbIn -- compile failure ("does not name a type"),
plus several call sites (Ft8UsbBridge_SetTxActive at
MainBoard_AudioIO.cpp:379/402, Ft8UsbBridge_Init at
MainBoard_AudioIO.cpp:570/Globals.cpp:518, Ft8UsbBridge_DrainToUSB at
MainBoard_AudioIO.cpp:575) called into functions that wouldn't even be
defined in that configuration -- link failures waiting to happen.

Restructured Ft8UsbBridge.cpp: the RX FIFO plumbing, Ft8UsbBridge_PutRxSamples,
Ft8UsbBridge_DrainToUSB (its only interface-dependent line already gated by
T41_CORE_HAS_USB_AUDIO_PUSH_BLOCK), and Ft8UsbBridge_SetTxActive never
touched AudioInputUSB/AudioOutputUSB in the first place, so they're now
defined unconditionally -- fixing every mismatched caller above without
touching the callers themselves. Only Ft8UsbBridge_Init/Ft8UsbBridge_GetSamples
and the AudioInputUSB/AudioRecordQueue/AudioConnection objects they depend on
stay behind the strict #if defined(T41_USB_AUDIO) && (defined(USB_AUDIO) ||
defined(USB_MIDI_AUDIO_SERIAL)) guard, with safe stubs (silence/no-op) in
the #else so callers don't need to know which USB Type is selected.

Audited every reference to AudioOutputUSB, AudioInputUSB, usb_audio_, and
Ft8UsbBridge_* under src/: CAT.cpp/Loop.cpp/MainBoard_DisplayHome.cpp only
call the always-defined GetFt8Mode()/SetFt8Mode(), and MainBoard_AudioIO.cpp's
Q_usbOut_L/R/usbRxGainL/R (AudioPlayQueue/AudioAmplifier) don't require
AUDIO_INTERFACE at all -- none of those needed changes.