Improve Engine: rename project, add Cratis packages, introduce ConceptAs types, and model Constraints

[Copilot]

• Rename VerticalSlices.Engine.csproj to Engine.csproj and update all references
• Add Cratis.Fundamentals, Cratis.Chronicle, and Cratis.Arc to Directory.Packages.props and Engine.csproj
• Create ConceptAs<> implementations for all primitive string types
• Add ConstraintType enum; update domain records to use concept types; enhance Constraint
• Remove redundant static implicit operator T(ConceptAs<T>) from all concept files — already provided by ConceptAs<> base class
• Update concept instructions to clarify the base class already provides the ConceptAs<T> → T conversion

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[einari]

@copilot Great. The operator overload for converting to the primitive type is implemented in the base type ConceptAs<>, so this is not needed. Add this to the concept instructions.

[Copilot]

@copilot Great. The operator overload for converting to the primitive type is implemented in the base type ConceptAs<>, so this is not needed. Add this to the concept instructions.

Removed the redundant ConceptAs<T> → T implicit operators from all concept files in 66ca434. Also updated the concept instructions to clarify that ConceptAs<T> already provides both directions of conversion, so only the string → ConceptXxx operator needs to be defined explicitly.