Add system parameter requests & model state spec

[TimothyWillard]

Added SystemABC.requested_parameters()/model_state() to allow systems
to provide information about their underlying state specification,
including shape, as well as request parameters that comply with said
requirements. Also integrated this work into the existing wrapper system
to demonstrate the functionality.

• Added requested_parameters()/model_state() methods to SystemABC.
• Extended wrap()/_AdapterSystem to accept explicit parameter
  request and model state callbacks.
• Updated _checked_partial() so already materialized ParameterValue
  instances are preserved when binding static parameters.
• Simplified the documentation quick start wrapper config to the minimal
  valid example.

[jc-macdonald]

LGTM — one non-blocking note on int coercion.

[jc-macdonald]

int-annotated stepper params are conventionally counts/sizes, so values should be whole numbers — but nothing upstream rejects a config like value: 3.7 since FixedParameter doesn't know the stepper's expected type. Ideally this validation would live in the Pydantic layer (e.g. ParameterRequest carrying a dtype hint so sample() can reject non-integer values at config time), but until that link exists, a guard here would catch the mistake:

if annotation is int and scalar != int(scalar):
    msg = (
        f"Parameter '{name}' is annotated as int "
        f"but received non-integer value {scalar}."
    )
    raise TypeError(msg)
return annotation(scalar)

Consider filing an issue to track the upstream fix.

[TimothyWillard]

Yeah, that's a good element to add here for now. I would also in the future like to add some richness to ParameterValue with typing info, so float vs int is explicit. Also adding in other common cases like positive, negative, constrained to (0,1), etc.