[FEATURE] Sequential model

[sdatkinson]

Summary

• add Sequential model loading and processing through the normal Core model factory
• define the canonical envelope as complete nested child NAM objects in config.models, with an empty top-level weights array and the existing file version
• support nested Sequential models and validate child sample rates, channel compatibility, wrapper weights, and processing buffer sizes
• document the format and the intentional rejection of the unrecoverable legacy prototype representation

Compatibility note

This does not change the existing .nam file version. Earlier experimental Sequential exports that stored bare child configs plus flattened wrapper weights are intentionally unsupported because they do not contain enough information to recover child architectures and weight boundaries reliably.

This PR also leaves the existing Core Linear tap-order convention unchanged. Trainer/Core agreement for multi-tap Linear models is a separate compatibility concern and is not silently folded into Sequential support.

Resolves #142