PD4 display test: cycle diagnostic patterns, not just all-on

The real display's power-on test (JP1 jumper 5, firmware $B888) runs through
several drawing sequences — a solid fill, a border+grid, more — each held
briefly, to expose dead dots and addressing faults; it isn't just full on/off.

Both vPLASMA and the replica firmware now cycle 5 representative patterns:
solid, border+grid, horizontal stripes, vertical stripes, checkerboard.
- VPlasmaDevice.ShowTestPattern(int) + TestPatternCount; the app cycles them
  on a 1.2s timer while jumper 5 is installed.
- PlasmaDisplay::showTestPattern(int) + TEST_PATTERN_COUNT (shared logic); the
  Matrix Portal sketch's DOWN button toggles the cycling test.

(The firmware's exact byte patterns live in the display's native scan-buffer
layout, which we haven't mapped, so these stand in for that sequence rather
than reproducing it byte-for-byte.)

Verified: 29 C# tests pass; the C++ patterns render distinctly on the host.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/VPlasma.App/MainForm.cs

@@ -30,8 +30,10 @@ internal sealed class MainForm : Form
 
     private readonly System.Windows.Forms.Timer _uiTimer = new() { Interval = 500 };
     private readonly System.Windows.Forms.Timer _demoTimer = new() { Interval = 2800 };
+    private readonly System.Windows.Forms.Timer _testTimer = new() { Interval = 1200 };
     private int _selfTestPage;
     private int _demoScreen;
+    private int _testPattern;
 
     // ---- config panel controls -------------------------------------------
     private readonly ComboBox _portBox = new() { Width = 168, DropDownStyle = ComboBoxStyle.DropDownList };
@@ -118,6 +120,7 @@ internal sealed class MainForm : Form
         _jp5.CheckedChanged += (_, _) => ApplyTestPattern();
         _jp6.CheckedChanged += (_, _) => ApplyDemo();
         _demoTimer.Tick += (_, _) => StepDemo();
+        _testTimer.Tick += (_, _) => StepTestPattern();
 
         // Default straps: jumper 2 installed = 9600 (the game's rate). Jumper 4
         // unstrapped = horizontal (the normal cockpit orientation).
@@ -130,6 +133,7 @@ internal sealed class MainForm : Form
         {
             _uiTimer.Dispose();
             _demoTimer.Dispose();
+            _testTimer.Dispose();
             _service.Dispose();
             _pipeService.Dispose();
         };
@@ -288,16 +292,25 @@ internal sealed class MainForm : Form
     {
         if (_jp5.Checked)
         {
-            _device.ShowTestPattern();
-            PrependLog("Jumper 5 installed — panel test pattern (all dots lit)");
+            PrependLog("Jumper 5 installed — panel test (cycling diagnostic patterns)");
+            _testPattern = 0;
+            _device.ShowTestPattern(0);
+            _testTimer.Start();
         }
         else
         {
+            _testTimer.Stop();
             _device.Reset();
             PrependLog("Jumper 5 removed — test pattern cleared");
         }
     }
 
+    private void StepTestPattern()
+    {
+        _testPattern = (_testPattern + 1) % VPlasmaDevice.TestPatternCount;
+        _device.ShowTestPattern(_testPattern);
+    }
+
     private void ApplyDemo()
     {
         if (_jp6.Checked)