Enable TreatWarningsAsErrors and analyzers repo-wide

[erlendellefsen]

The build does not enforce warnings or analyzers. For a published library this is the wrong default. Land this last so the cleanup PRs are already in.

Acceptance criteria

• Directory.Build.props sets TreatWarningsAsErrors=true, EnableNETAnalyzers=true, EnforceCodeStyleInBuild=true, AnalysisMode=Recommended (or stricter)
• All resulting warnings either fixed or explicitly suppressed in .editorconfig with a justification
• CI build is green
• Test project may opt out if generated-code noise is unmanageable; document any opt-out