Implement StateLayout pack, unpack, and validation infrastructure

[bernalde]

Parent: #118

Goal

Introduce a small StateLayout abstraction in Commons.py as the single owner of state-vector slice offsets.

Acceptance criteria

• Build the layout from existing state metadata.
• Provide packing, zero-copy unpacking, and explicit shape validation.
• Keep the flat vector at the Assimulo/SUNDIALS boundary.
• Add focused unit tests for scalar, vector, and discretized state blocks.
• Preserve compatibility with existing unpacking helpers during incremental adoption.