Design for PowerFlow and PhasorDynamics Interoperability

[lukelowry]

Description

Partially addresses #466, #405, and allow us to close related PR #391

Introduce .state.json as a shared initialization/state artifact for PowerFlow and PhasorDynamics (and compatible with EMT), separating operating state and dispatch from the static .case.json model definition. This provides a file-based interoperability boundary without coupling the model implementations.

The proposed format is documented in STATE.md. Further justification for the design is provided in that document

Proposed changes

• Document the proposed state-file structure and migration path.
• Store bus voltages and device current injections under buses.
• Store device dispatch, connectivity, tap, phase, and other initialization states under devices.
• Add a Hawaii example at nominal steady-state dispatch.

If approved, I would like to include a *.state.json for each case as an initial artifact. I only committed an example for the Hawaii case.

[lukelowry]

Using this approach its very easy to exchange states between PowerFlow, PhasorDynamics, and OPF (experimental), see this downstream branch. I can reliably start in steady state after changing the generator dispatch using OPF! And the only shared knowledge between the OPF/ and PhasorDynamics/ is the proposed *.state.json. This contract would make initialization of EMT much easier (which is notoriously hard)

[lukelowry]

Drafting since not likely to be done before release but is very important for the development of EMT and practical use.