refactor: implement StronglyTypedId for entity identifiers

[mcj-coder]

Summary

Refactor existing entity identifier types to use the StronglyTypedId source generator library instead of manual implementations.

Library

https://github.com/andrewlock/StronglyTypedId

Current State

Currently using manual readonly record struct implementations for IDs like CharacterId.

Proposed Changes

• Add StronglyTypedId NuGet package
• Refactor CharacterId to use [StronglyTypedId] attribute
• Apply same pattern to any future entity IDs
• Remove manual ID boilerplate code

Benefits

• Less boilerplate code
• Consistent ID handling patterns
• Built-in JSON/EF Core converters
• Compile-time type safety

Affected Types

• CharacterId - currently manual implementation in src/FantasyRpgWorld.Core/Domain/ValueObjects/CharacterId.cs
• Any future entity identifiers

Acceptance Criteria

• StronglyTypedId package added
• CharacterId refactored to use attribute
• All tests continue to pass
• Documentation updated if needed

Labels

type:refactor