Root cause: Oboe capture at 48kHz with InputPreset::VoiceCommunication
cannot open against a BT SCO device (only supports 8/16kHz). The stream
silently falls back to builtin mic, delivering zeros.
Fix: add bt_active flag to WzpOboeConfig. When set, capture skips
setSampleRate and setInputPreset, letting the system route to BT SCO
at its native rate. Oboe's SampleRateConversionQuality::Best resamples
to 48kHz for our ring buffers. Playout uses Usage::Media in BT mode.
New API: wzp_native_audio_start_bt() for BT mode, called from
set_bluetooth_sco(on=true). Normal audio_start() restores the
standard config when switching back to earpiece/speaker.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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