Support C# 14 extension members in CsSig

[agocke]

Summary

Projects using C# 14 extension members (extension(Receiver) { ... }) produced broken .cssig files: each block was emitted as a nameless public class { ... }. Roslyn models each block as a nested type with an unspeakable, compiler-generated name, which the writer rendered as an empty class name.

Changes

• ExtensionMembers helper — recognises extension marker types and their receivers via INamedTypeSymbol.IsExtension / ExtensionParameter / TypeKind.Extension.
• Writer — emits proper extension<T>(receiver) { ... } blocks (deterministically ordered), rendering the receiver with ref-kind and nullability.
• Signature model — keys extension markers by a fixed <extension> discriminator plus the receiver type, so distinct blocks sharing a receiver are not conflated (soundness).
• API surface — no longer double-counts the implicit implementation methods the compiler synthesises on the host static class; members are tracked through the extension marker types.
• Roslyn 5.0 — bump the analyzer/code-fix projects from the Microsoft.CodeAnalysis 4.0 baseline to 5.0 (extension members require it), removing the previous reflection/numeric-enum workarounds and the now-stale (RefKind)4 fallback. RS1035/RS1037 (newly enforced) are suppressed via a documented global config.
• Tests — bump the harness to Roslyn 5.x (Microsoft.CodeAnalysis.Analyzer.Testing 1.1.4, CSharp.Workspaces 5.3.0) and add round-trip + soundness regression tests.

Validation

• Round-trip (generate → analyze) on extension members: 0 diagnostics.
• Missing / extra extension members each report exactly one CSSIG001/CSSIG002.
• Regenerated a real-world PublicAPI.cssig (serde): all extension(...) blocks correct, no nameless classes, nullable receivers preserved.
• Full suite: 94 passed, 1 skipped.