fix(vae-roundtrip): pass bigvgan path to encoder-only FeaturesUtils

AutoEncoderModule unconditionally asserts vocoder_ckpt_path is not None
even when need_vae_encoder=True. Pass best_netG.pt to satisfy the assert;
the vocoder weights are not actually used since decode+vocode go through
model["feature_utils"].

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

nodes/selva_vae_roundtrip.py

@@ -63,13 +63,17 @@ class SelvaVaeRoundtrip:
             )
 
         # Load encoder only — decoder/vocoder come from model["feature_utils"]
-        # to mirror exactly what the sampler uses
+        # to mirror exactly what the sampler uses.
+        # AutoEncoderModule requires vocoder_ckpt_path even when only encoding,
+        # so pass the BigVGAN path (weights won't actually be used for decode here).
+        bigvgan_path = _SELVA_DIR / "ext" / "best_netG.pt"
         print("[VAE Roundtrip] Loading VAE encoder...", flush=True)
         vae_enc = FeaturesUtils(
             tod_vae_ckpt=str(vae_path),
             enable_conditions=False,
             mode=mode,
             need_vae_encoder=True,
+            bigvgan_vocoder_ckpt=str(bigvgan_path) if bigvgan_path.exists() else None,
         ).to(device).eval()
 
         try: