Several code style rules we agreed on can be turned on in .editorconfig with little
or no reformatting. Grouping them keeps the per-rule enablement PRs small.
Acceptance criteria:

• csharp_prefer_braces set to when_multiline:warning (CS-CF-02).
• csharp_preferred_modifier_order set to warning, and the 36 affected files fixed
  with dotnet format (CS-TV-02).
• csharp_prefer_simple_using_statement set to true:suggestion (CS-CF-08).
• An interface naming rule added at suggestion (CS-NM-04).
• The index/range operator keys flipped to true:none (CS-EL-11).
• dotnet format --verify-no-changes stays green.