feat(A.2 #19): progressive WAV write during live capture

The Vec<f32> in-memory accumulator on run_live_session had three
failure modes: (a) a crash during transcription took the recording
with it, (b) RAM grew linearly with session length, (c) OOM killed
the capture thread silently.

New kon_audio::WavWriter wraps hound::WavWriter<BufWriter<File>> with
an append-friendly API and a 500 ms-granularity header flush. On any
abort after a flush the on-disk file is a valid, playable WAV. Unit
test (brief item #19 acceptance) simulates the abort with
std::mem::forget and asserts the pre-flush samples are recoverable.

Live capture now:
- resolves the destination path at start_live_transcription_session
  time via a new resolve_recording_path helper extracted from
  persist_audio_samples,
- opens a WavWriter before any samples arrive, sample rate taken from
  LocalEngine::capabilities() (#13 wiring) with 16 kHz fallback,
- feeds the resampler output through WavWriter::append inside
  append_resampled_audio — drops the writer with a user-visible
  warning if a write fails mid-session,
- calls flush() at stop (after resampler tail), finalise() on clean
  exit, and drops-to-last-flushed state on abort.

LiveSessionSummary.audio_samples → audio_path: the path is already
written by the time stop_live_transcription_session runs; no
post-session write step remains for live capture.
persist_audio_samples is kept for the offline save_audio command.

crates/audio/src/lib.rs

@@ -12,4 +12,4 @@ pub use decode::decode_audio_file;
 pub use resample::resample_to_16khz;
 pub use streaming_resample::StreamingResampler;
 pub use vad::SpeechDetector;
-pub use wav::{read_wav, write_wav};
+pub use wav::{read_wav, write_wav, WavWriter};