refactor: fix analyzer findings and add header comments

Fixes every finding the new quality gate reports, without changing behavior:

- One type per file (MA0048): AppBrand, Attribution, NullActivityLog,
  LogSnapshot, ISanitizable, AppTheme, UpdateCheckResult, UpdateText,
  IUpdateBackend, ContentDialogPrompter, WindowExtensions, MessageBoxPrompter,
  and the samples' SampleSettings/SampleSettingsContext get their own files
  (the samples now link samples/Shared/*.cs).
- Regex match timeouts on the two generated regexes (MA0009).
- AppInfo.Id reports nameof(value) as the parameter (MA0015); test updated.
- Splash dismissal is a named DismissAfterAsync started through the
  dispatcher (MA0147, MA0134), and the WinUI splash constructor is split into
  builders (MA0051). The WPF countdown must go through the dispatcher: the
  sample builds its window before Application.Run, when no synchronization
  context exists.
- The WinUI sample's startup update check is a named method (MA0147).
- CA1859 in BrandTool and the version tests; private static readonly fields
  renamed to s_camelCase (IDE1006).
- A header comment on every type and method (K22-CODE-09/CS-04): hand-written
  for library, sample, and tool code; test methods summarize their
  Subject_Behavior names.

Verified: zero-warning x64 build, 115 unit tests with 100% line coverage of
KofTwentyTwo.AppKit and KofTwentyTwo.AppKit.Updates, 12 UI tests, and
dotnet format --verify-no-changes --severity warn.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: James Maes <james@kof22.com>

samples/AppKit.Sample.WinUI/Program.cs

@@ -15,6 +15,7 @@ namespace AppKit.Sample.WinUI;
 /// </summary>
 public static class Program
 {
+   /// <summary>Runs Velopack&apos;s install and update hooks first, then starts the WinUI application (the XAML-generated Main is disabled for this).</summary>
    [STAThread]
    private static void Main()
    {