1.2.0: Add supported.enable option

Changes

• BREAKING: Change horizon.systems.supported to horizon.systems.supported.targets
• Add horizon.systems.supported.enable option that sets the top-level systems attribute
• Update test flake to use new structure with horizon.systems.supported.enable = true

Motivation

This change provides a cleaner way to enable horizon supported systems while keeping the option structure consistent with the CI configuration. Users can now simply set horizon.systems.supported.enable = true; instead of manually setting systems = config.horizon.systems.supported;.

Testing

• Test flake evaluates successfully with all 4 supported systems
• Main flake evaluates successfully

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