Spec modules

This umbrella re-exports shape-indexed modules built from TorchLean's mathematical layer and model definitions. A Spec.Module α σ τ is a pure map from tensors of shape σ to tensors of shape τ; Spec.Module.Chain composes modules only when adjacent shapes agree.

The forward field carries the semantics. Operation names and Python expressions are metadata for reports and source export, not part of the mathematical definition.