fix(bluetooth): BT SCO mode skips 48kHz + VoiceCommunication on capture

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>

crates/wzp-native/cpp/oboe_bridge.h

@@ -16,6 +16,7 @@ typedef struct {
     int32_t sample_rate;
     int32_t frames_per_burst;
     int32_t channel_count;
+    int32_t bt_active;  /* nonzero = BT SCO mode: skip sample rate + input preset */
 } WzpOboeConfig;
 
 typedef struct {