Sequential GraphSpec

This is the canonical import for GraphSpec's sequential model language. It exposes:

• the extensible Primitive interface and shape-indexed Chain syntax;
• standard linear, ReLU, and softmax chain constructors;
• direct pure tensor semantics through Interp.spec;
• execution-polymorphic lowering through Chain.toProgram;
• structural conversion to the canonical GraphSpec DAG representation.

The implementation is organized by ownership in Chain.Syntax, Chain.Primitives, Chain.Semantics, Chain.Lowering, and Chain.ToDAG. Existing users should continue to import NN.GraphSpec.Core.

For skip connections, shared intermediates, residual additions, and other multi-input nodes, use NN.GraphSpec.DAG.Core directly.