Prove the first Behavior-native OS vertical slice

[LeftTwixWand]

Outcome

Deliver a reproducible end-to-end Behavior-native OS slice from local startup through verified interaction.

Acceptance criteria

• Start from a clean clone.
• Launch the system with aspire run.
• Open the Flutter shell.
• Activate a shipped concrete Behavior Neuron.
• Call an explicitly configured model Neuron.
• Render a typed Surface.
• Pass the Behavior's BDD scenarios.

Non-goals

• Expanding the slice beyond the required shell, Behavior, model call, Surface, and BDD journey.