Conversion of sequential GraphSpec chains to DAGs

This is the canonical import for structural chain-to-DAG conversion and the associated DAG model constructors. Term construction lives in Chain.ToDAG.Core; parameter initialization and model packaging live in Chain.ToDAG.Model. Chain.ToDAG.Semantics proves that the converted term has the direct chain interpretation for every parameter pack and input, including custom primitives. This is a pure-semantics theorem; executable primitive programs require separate agreement proofs.