feat: spectral metrics per eval sample in experiment summary

Computes hf_energy_ratio (>4kHz), spectral_centroid_hz, spectral_rolloff_hz
at each save_every checkpoint. Logged to console and stored in
experiment_summary.json under results.spectral_metrics[step].

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

nodes/selva_lora_scheduler.py

@@ -429,6 +429,7 @@ class SelvaLoraScheduler:
                 duration          = time.monotonic() - t_start
                 loss_history      = r["loss_history"]
                 grad_norm_history = r.get("grad_norm_history", [])
+                spectral_metrics  = r.get("spectral_metrics", {})
                 run_start_step    = r.get("start_step", 0)
                 smoothed          = _smooth_losses(loss_history) if loss_history else []
 
@@ -460,6 +461,7 @@ class SelvaLoraScheduler:
                     ),
                     "loss_history":         [round(v, 6) for v in loss_history],
                     "grad_norm_history":    grad_norm_history,
+                    "spectral_metrics":     {str(k): v for k, v in spectral_metrics.items()},
                     "log_interval":         log_interval,
                     "duration_seconds":     round(duration, 1),
                 }

nodes/selva_lora_trainer.py

@@ -159,6 +159,40 @@ _SPEC_DB_FLOOR = -80.0
 _SPEC_LOG_BINS = 256
 
 
+def _spectral_metrics(wav: torch.Tensor, sr: int) -> dict:
+    """Compute spectral quality metrics for a mono [1, L] float32 CPU tensor.
+
+    Returns:
+      hf_energy_ratio   — energy above 4kHz / total energy  (low bitrate → low value)
+      spectral_centroid_hz — energy-weighted mean frequency
+      spectral_rolloff_hz  — frequency below which 85% of energy sits
+    """
+    import numpy as np
+    wav_np  = wav.squeeze(0).numpy()
+    hop     = min(_SPEC_HOP, _SPEC_N_FFT)
+    window  = torch.hann_window(_SPEC_N_FFT)
+    stft    = torch.stft(torch.from_numpy(wav_np), n_fft=_SPEC_N_FFT, hop_length=hop,
+                         window=window, return_complex=True)
+    power   = stft.abs().pow(2).mean(dim=1).numpy()   # [n_freqs] averaged over time
+    freqs   = np.linspace(0, sr / 2, len(power))
+
+    total   = power.sum() + 1e-12
+    hf_mask = freqs >= 4000
+    hf_ratio = float(power[hf_mask].sum() / total)
+
+    centroid = float((freqs * power).sum() / total)
+
+    cumsum  = np.cumsum(power)
+    rolloff_idx = np.searchsorted(cumsum, 0.85 * cumsum[-1])
+    rolloff = float(freqs[min(rolloff_idx, len(freqs) - 1)])
+
+    return {
+        "hf_energy_ratio":    round(hf_ratio, 4),
+        "spectral_centroid_hz": round(centroid, 1),
+        "spectral_rolloff_hz":  round(rolloff, 1),
+    }
+
+
 def _save_spectrogram(wav: torch.Tensor, sr: int, path: Path) -> None:
     """Save a log-frequency dB spectrogram PNG for an eval sample.
 
@@ -632,6 +666,7 @@ class SelvaLoraTrainer:
         loss_history      = []
         running_loss      = 0.0
         grad_norm_history = []
+        spectral_metrics  = {}   # {step: {hf_energy_ratio, spectral_centroid_hz, spectral_rolloff_hz}}
         running_grad_norm = 0.0
         grad_norm_count   = 0
 
@@ -751,9 +786,14 @@ class SelvaLoraTrainer:
                             sf.write(str(wav_path), wav.squeeze(0).numpy(), sr)
                         print(f"[LoRA Trainer] Sample saved: {wav_path}", flush=True)
                         try:
+                            metrics = _spectral_metrics(wav, sr)
+                            spectral_metrics[step] = metrics
+                            print(f"[LoRA Trainer] Spectral: hf_ratio={metrics['hf_energy_ratio']:.3f}  "
+                                  f"centroid={metrics['spectral_centroid_hz']:.0f}Hz  "
+                                  f"rolloff={metrics['spectral_rolloff_hz']:.0f}Hz", flush=True)
                             _save_spectrogram(wav, sr, wav_path)
                         except Exception as e:
-                            print(f"[LoRA Trainer] Spectrogram failed: {e}", flush=True)
+                            print(f"[LoRA Trainer] Spectral/spectrogram failed: {e}", flush=True)
 
                 last_step = step
                 pbar_train.update(1)
@@ -802,6 +842,7 @@ class SelvaLoraTrainer:
             "loss_curve":        loss_curve,
             "loss_history":      loss_history,
             "grad_norm_history": grad_norm_history,
+            "spectral_metrics":  spectral_metrics,
             "start_step":        start_step,
             "meta":              meta,
             "completed":         True,