chore: vendor selva_core from jnwnlee/selva@d7d40a9

Pure PyTorch SelVA source for SelvaModelLoader/FeatureExtractor/Sampler nodes.
Imports rewritten from selva.* to selva_core.*. mel_converter.py: replaced
librosa.filters.mel with pure-numpy implementation to avoid librosa→numba→NumPy
version incompatibility in some ComfyUI environments.

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

selva_core/model/utils/sample_utils.py

@@ -0,0 +1,12 @@
+from typing import Optional
+
+import torch
+
+
+def log_normal_sample(x: torch.Tensor,
+                      generator: Optional[torch.Generator] = None,
+                      m: float = 0.0,
+                      s: float = 1.0) -> torch.Tensor:
+    bs = x.shape[0]
+    s = torch.randn(bs, device=x.device, generator=generator) * s + m
+    return torch.sigmoid(s)