feat(sdf): S12-C PyTorch integration + Python bindings

Add batch evaluation/gradient functions for SDF trees (sdf_torch.h/cpp).
Add sdf_gradient.h with finite-difference gradient utilities.
Add Python modules vde.sdf (high-level SDF API) and vde.torch_sdf (PyTorch autograd).
Add test_sdf_torch_bridge.cpp with batch eval/gradient tests.
Integrate into vde_sdf library and test suite.

src/CMakeLists.txt

@@ -171,6 +171,7 @@ add_library(vde_sdf STATIC
     sdf/sdf_primitives.cpp
     sdf/sdf_tree.cpp
     sdf/sdf_to_mesh.cpp
+    sdf/sdf_torch.cpp
 )
 target_include_directories(vde_sdf
     PUBLIC  ${CMAKE_SOURCE_DIR}/include

src/sdf/sdf_torch.cpp

@@ -0,0 +1,44 @@
+#include "vde/sdf/sdf_torch.h"
+#include "vde/sdf/sdf_gradient.h"
+
+namespace vde::sdf {
+
+void evaluate_batch(const SdfNodePtr& root,
+                    const double* points, int n,
+                    double* distances) {
+    auto fn = [&](const Point3D& p) -> double { return evaluate(root, p); };
+    for (int i = 0; i < n; ++i) {
+        Point3D p(points[i*3], points[i*3+1], points[i*3+2]);
+        distances[i] = fn(p);
+    }
+}
+
+void gradient_batch(const SdfNodePtr& root,
+                    const double* points, int n,
+                    double* gradients) {
+    auto fn = [&](const Point3D& p) -> double { return evaluate(root, p); };
+    for (int i = 0; i < n; ++i) {
+        Point3D p(points[i*3], points[i*3+1], points[i*3+2]);
+        Vector3D g = gradient(fn, p);
+        gradients[i*3] = g.x();
+        gradients[i*3+1] = g.y();
+        gradients[i*3+2] = g.z();
+    }
+}
+
+void evaluate_with_gradient_batch(const SdfNodePtr& root,
+                                   const double* points, int n,
+                                   double* distances,
+                                   double* gradients) {
+    auto fn = [&](const Point3D& p) -> double { return evaluate(root, p); };
+    for (int i = 0; i < n; ++i) {
+        Point3D p(points[i*3], points[i*3+1], points[i*3+2]);
+        auto [val, grad] = evaluate_with_gradient(fn, p);
+        distances[i] = val;
+        gradients[i*3] = grad.x();
+        gradients[i*3+1] = grad.y();
+        gradients[i*3+2] = grad.z();
+    }
+}
+
+} // namespace vde::sdf