Tensor API

Umbrella import for the small tensor subchapter:

• Core: the shape-indexed tensor datatype and accessors;
• Constructors: total tensor builders;
• Linalg: matrix/vector primitives; and
• Vec: the vector-specialized Tensor α (.dim n .scalar) interface.

Elementwise operations, reductions, broadcasting, flattening, and shape-changing helpers live one level up in NN.Spec.Core.TensorOps and NN.Spec.Core.TensorReductionShape.