Define exact provider Neuron identities and account bindings

[LeftTwixWand]

Outcome

Make provider selection explicit, stable, permissioned, and testable through typed Neuron identities and bindings.

Acceptance criteria

• Represent each model and major version as a distinct identity.
• Prevent silent substitution between identities.
• Return typed unavailability and failure outcomes.
• Support named account bindings.
• Require permission checks for rebinding.
• Provide deterministic test doubles.

Non-goals

• Automatically substituting models, versions, or account bindings.