fix: restore CrtDisplay.PositiveMod accessibility for unit tests

A previous code-review pass changed PositiveMod to private static,
breaking CrtDisplayTests which call it directly via CrtDisplay.PositiveMod.

Fix:
- private static → internal static  (keeps it off the public API)
- Add Properties/AssemblyInfo.cs with [InternalsVisibleTo("Pipboy.Avalonia.Tests")]
  so the test project can access internal members without any visibility leaks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: NeverMorewd

src/Pipboy.Avalonia/Controls/CrtDisplay.cs

@@ -809,7 +809,7 @@ private void EnsureFlickerBrushes()
     /// <summary>
     /// Always-positive modulo. Returns 0 when <paramref name="m"/> is zero or negative.
     /// </summary>
-    private static double PositiveMod(double x, double m)
+    internal static double PositiveMod(double x, double m)
     {
         if (m <= 0) return 0;
         double r = x % m;

src/Pipboy.Avalonia/Properties/AssemblyInfo.cs

@@ -0,0 +1,4 @@
+using System.Runtime.CompilerServices;
+
+// Allow the unit test project to access internal members (e.g. CrtDisplay.PositiveMod).
+[assembly: InternalsVisibleTo("Pipboy.Avalonia.Tests")]