Canonical GraphSpec DAG

This is the canonical import for GraphSpec's typed SSA/DAG representation. It exposes:

• shape-indexed DAG syntax, substitutions, and shared multi-result blocks;
• pure tensor semantics and preservation theorems;
• execution-polymorphic TorchLean lowering;
• single- and multi-output model wrappers.

The implementation is organized by concept in DAG.Syntax, DAG.Semantics, DAG.Lowering, and DAG.Model. Most users should continue to import NN.GraphSpec.DAG.Core.