Revive components

TL;DR Revive and flesh out the Component Protocol from model/common/src/icon4py/model/common/components/components.py into a usable, well-documented interface for model building blocks.

Problem / motivation

The codebase already defines a Component Protocol (components.py) that declares the shape a model component should have: typed inputs and outputs with CF metadata, and a __call__ that transforms model state. However, it is currently a stub: the abstract methods (inputs_properties, outputs_properties) are unimplemented, the open TODOs are unresolved, and no concrete components in the codebase adopt the protocol.

Without a concrete protocol to follow, component code drifts into ad-hoc interfaces, making it harder to compose, test, and validate model building blocks.

Proposal

A stub. The intent is to take the existing skeleton and:

1. Resolve the open TODOs (unit-consistency hooks, dimension checks, state interface improvements)
2. Provide a reference implementation that concrete components can inherit from
3. Define how components declare and validate their input/output contracts at runtime or type-check time

Alternatives considered

Not yet evaluated. This is a placeholder to start the discussion.

Open questions / conflicts

• How does this relate to the existing driver orchestration? Should components be independently runnable, or only as part of a composed pipeline?
• The TODO in components.py:74 questions whether outputs should be split by type (tendencies, diagnostics, prognostics). This needs a design decision before the protocol is finalized.
• The TODO in components.py:97 asks whether passing the entire state is the right interface. Alternatives include scoped state views or explicit field descriptors.

Appendices

• SPEC reference — frozen specification from one exploration of a concrete Component interface, included as an additional starting point. Unreviewed; not yet agreed.