fix: switch to VideoPrism large (1024-dim) and fix Synchformer output shape

prismaudio.json conditioner config requires:
- video_features: dim=1024 → switch videoprism_public_v1_base → large (ViT-L)
- sync_features: dim=768, length divisible by 8 → expand [num_seg,768] to
  [num_seg*8,768] (per-frame) so Sync_MLP can reshape by groups of 8

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

data_utils/v2a_utils/feature_utils_288.py

@@ -70,9 +70,9 @@ class FeaturesUtils:
             return
         from videoprism import models as vp
         import jax
-        print("[FeaturesUtils] Loading VideoPrism...")
-        self._vp_model = vp.get_model("videoprism_public_v1_base")
-        self._vp_state = vp.load_pretrained_weights("videoprism_public_v1_base")
+        print("[FeaturesUtils] Loading VideoPrism large (1024-dim, required by prismaudio conditioner)...")
+        self._vp_model = vp.get_model("videoprism_public_v1_large")
+        self._vp_state = vp.load_pretrained_weights("videoprism_public_v1_large")
         self._jax_forward = jax.jit(
             lambda x: self._vp_model.apply(self._vp_state, x, train=False)
         )
@@ -233,5 +233,7 @@ class _SynchformerVisualEncoder(nn.Module):
             # Mean-pool over frames and spatial dims → [C*H*W] → project
             pooled = chunk.mean(dim=0).reshape(-1)  # [C*H*W]
             feat = self._linear(pooled.unsqueeze(0))  # [1, dim]
-            segs.append(feat)
-        return torch.cat(segs, dim=0)  # [num_seg, 768]
+            # Repeat feature once per frame so output is [num_seg*8, dim]
+            # Sync_MLP expects per-frame features grouped in 8-frame segments
+            segs.append(feat.expand(seg, -1))
+        return torch.cat(segs, dim=0)  # [num_seg*8, 768]