Emit Roslyn diagnostic when unsupported types are silently skipped

[jonathanpeppers]

Both [SortingNetwork] and [HybridSortingNetwork] generators silently skip unsupported element types (e.g., ypeof(decimal)) without any compiler warning or error. This can be confusing for users who expect code to be generated but get nothing.

Proposal

Emit a Roslyn diagnostic (warning or info) when an attribute specifies a type that is not in the supported set. For example:

[SortingNetwork(typeof(decimal))] // SN0001: 'decimal' is not a supported sorting network element type
partial class MySorter { }

This should apply to both attribute pipelines:

• SortingNetworkAttribute
• HybridSortingNetworkAttribute

Context

Discussed in PR #108 code review: #108 (comment)