fix: preserve original audio extension when copying reference file

shutil.copy2 was writing FLAC binary to reference.wav — unplayable.
Now copies as reference{.flac/.wav/etc} matching the source extension.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/selva_lora_evaluator.py

@@ -78,8 +78,7 @@ def _draw_metric_comparison(adapter_ids: list, metrics_list: list, output_path:
         "#9B59B6", "#1ABC9C", "#E67E22", "#95A5A6",
     ]
 
-    n = len(adapter_ids)
-    fig = Figure(figsize=(12, max(4, n * 0.6 + 2)), dpi=110, tight_layout=True)
+    fig = Figure(figsize=(12, max(4, len(adapter_ids) * 0.6 + 2)), dpi=110, tight_layout=True)
     axes = [fig.add_subplot(2, 2, i + 1) for i in range(4)]
 
     for ax, (key, title) in zip(axes, METRICS):
@@ -212,8 +211,8 @@ class SelvaLoraEvaluator:
             try:
                 ref_wav = _load_audio(audio_path, seq_cfg.sampling_rate, seq_cfg.duration)
                 ref_wav = ref_wav.unsqueeze(0)   # [1, L]
-                ref_out = output_dir / "reference.wav"
                 import shutil
+                ref_out = output_dir / f"reference{audio_path.suffix}"
                 shutil.copy2(str(audio_path), str(ref_out))
                 ref_record["wav_path"]         = str(ref_out)
                 ref_record["spectral_metrics"] = _spectral_metrics(ref_wav, seq_cfg.sampling_rate)