Align C# styling with EncDotNet.S100

[philliphoff]

EncDotNet currently lacks the explicit style rules and CI enforcement used by EncDotNet.S100, allowing formatting and analyzer drift between the related repositories. This adopts the same standards so contributors and automation share a consistent baseline.

Summary

• add the shared C# .editorconfig rules and an EncDotNet-specific normative coding guide
• enforce warnings-as-errors, transitive NuGet auditing, whitespace formatting, and using-directive checks
• reformat the existing codebase and resolve warning-level naming/style violations
• update vulnerable Avalonia, MessagePack, and OpenTelemetry dependencies surfaced by the stricter audit policy
• update contributor-facing Copilot guidance and documentation navigation

Validation

• Release build succeeds with zero warnings
• all 469 unit tests pass
• CI-equivalent whitespace and using checks report no changes