Release v7.21.0

Added

• The cratis-fundamentals-concept skill teaches concepts that create their own value: implement IGeneratable<TSelf> and generate with GenerateValue.Uuid(), or GenerateValue.UuidV7() when index locality matters, from Cratis.Fundamentals 7.23.0. It covers the v7 trade-offs (reveals creation time, not monotonic within a millisecond) and when not to generate: client-supplied ids, secrets and tokens, and idempotency. (part of #576)
• Evaluations for cratis-fundamentals-concept: a self-generating identity, choosing UUID v7, and refusing generation where it does not fit. (part of #576)

Changed

• The AuthorId examples in the concepts and vertical-slices rules use IGeneratable<AuthorId> with GenerateValue.Uuid() instead of Guid.NewGuid(). (part of #576)