Add "Direct USB TX" toggle to bypass the mixer on transmit

[patrickrb]

What

Adds a Direct USB TX on/off toggle in Settings → Radio & Audio → Audio (right under Audio Output).

• On — TX audio goes straight to the USB radio over the direct libusb path, bypassing Android's shared mixer. Notification and other-app sounds can no longer be mixed into the transmission and go out on air.
• Off — normal AudioTrack output (system-default sink).

Why

The direct-libusb TX path already existed in FT8TransmitSignal (chosen when audioOutputDeviceId == -1 && usbAudioOutputVendorId != 0), but the only way to select it was to find the … (USB direct) row in the Audio Output device picker — sitting right next to the AudioManager USB Audio entry for the same card, hard to tell apart. The direct path is the robust fix for the mixer-bleed class of problem (other apps' sounds transmitted on air) that the advisory audio-focus work in #597 can only partially mitigate, since notification sounds ignore audio focus.

This exposes it as a clear, reversible toggle instead of touching the (working, timing-sensitive) TX pipeline.

How

• The toggle is a second view of the same audio-output selection: it reads and writes the exact three persisted config keys (audioOutputDevice, usbAudioOutputVid, usbAudioOutputPid) the device picker uses, so the two controls stay in sync and the setting round-trips across restarts.
• Enabling with no USB sound card attached refuses and toasts why; enabling requests USB permission just like the picker path.
• Decision logic extracted to DirectUsbTx.kt (isDirectUsbTxEnabled / directUsbTxSelection / systemDefaultTxSelection) — pure functions, unit-tested. The enable predicate deliberately mirrors the engine's TX-path branch condition so the two can't drift.

Testing

• New DirectUsbTxTest covers the on/off predicate (including the deviceId-is-−1-but-no-VID edge) and the round-trip of each selection back through the predicate. Passes.
• ./gradlew assembleDebug clean.
• Not yet exercised on-device (no phone attached this session). Scope is TX-path only; RX capture is a separate setting.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 12.96296% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.94%. Comparing base (d01be83) to head (edc8a1e).

Files with missing lines	Patch %	Lines
...dio/ks3ckc/ft8af/ui/settings/RadioAudioSettings.kt	0.00%	47 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev     #722      +/-   ##
============================================
- Coverage     35.98%   35.94%   -0.05%     
  Complexity      289      289              
============================================
  Files           247      248       +1     
  Lines         30276    30330      +54     
  Branches       3989     3994       +5     
============================================
+ Hits          10895    10902       +7     
- Misses        19032    19079      +47     
  Partials        349      349              

Flag	Coverage Δ
android	17.55% <12.96%> (-0.02%)	⬇️
native	9.93% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...tlin/radio/ks3ckc/ft8af/ui/settings/DirectUsbTx.kt	100.00% <100.00%> (ø)
...dio/ks3ckc/ft8af/ui/settings/RadioAudioSettings.kt	0.15% <0.00%> (-0.02%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.