[Feature] Define blueprint root YAML schema

[Rexus]

What

Define the root YAML schema for an Archflow blueprint.

This should establish the top-level structure for projects, services, networks, physical layout, and metadata.

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Why

Everything in Archflow depends on a stable YAML-first model. This is the foundation for parsing, validation, projections, generation, and UI editing.

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Done when

• A first root schema structure is defined
• Required top-level sections are identified
• A minimal valid example can be written against it