FakeClaude, test per area e renderer diviso in due file

- tests/FakeClaude: eseguibile finto 'claude' configurabile con variabili
  d'ambiente (registro di argomenti/cwd/stdin, output, exit code, ritardo),
  con le prove FK-01…FK-05.
- Test organizzati per livello (Gui/, Process/, Support/) e struttura
  descritta in TESTING.md.
- MarkdownView: la parte sulle immagini passa in MarkdownView.Images.cs
  (partial), senza cambiare il comportamento.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/agent-wiki-ui.Tests/Support/FakeClaudePath.cs

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+namespace AgentWikiUi.Tests;
+
+/// <summary>Percorso del <c>claude</c> finto compilato (tests/FakeClaude), nella stessa configurazione dei test.</summary>
+static class FakeClaudePath
+{
+    public static string Exe
+    {
+        get
+        {
+            // …/tests/agent-wiki-ui.Tests/bin/<config>/net10.0/
+            var tfm = new DirectoryInfo(AppContext.BaseDirectory.TrimEnd(Path.DirectorySeparatorChar));
+            var config = tfm.Parent!;
+            var testsRoot = config.Parent!.Parent!.Parent!;
+            return Path.Combine(testsRoot.FullName, "FakeClaude", "bin", config.Name, tfm.Name, OperatingSystem.IsWindows() ? "claude.exe" : "claude");
+        }
+    }
+}

tests/agent-wiki-ui.Tests/Support/PngFactory.cs

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+using System.IO.Compression;
+
+namespace AgentWikiUi.Tests;
+
+/// <summary>Scrive PNG minimali (colore pieno) senza dipendere da Avalonia.</summary>
+static class PngFactory
+{
+    public static byte[] Create(int width, int height)
+    {
+        var raw = new byte[height * (1 + width * 3)];
+        for (var y = 0; y < height; y++)
+            for (var x = 0; x < width; x++) raw[y * (1 + width * 3) + 1 + x * 3] = 200; // filtro 0 per riga, canale rosso
+        using var ms = new MemoryStream();
+        ms.Write([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]);
+        var ihdr = new byte[13];
+        WriteInt(ihdr, 0, width); WriteInt(ihdr, 4, height);
+        ihdr[8] = 8; ihdr[9] = 2; // 8 bit, RGB
+        Chunk(ms, "IHDR", ihdr);
+        using (var z = new MemoryStream())
+        {
+            using (var zs = new ZLibStream(z, CompressionLevel.Fastest, leaveOpen: true)) zs.Write(raw);
+            Chunk(ms, "IDAT", z.ToArray());
+        }
+        Chunk(ms, "IEND", []);
+        return ms.ToArray();
+    }
+
+    static void WriteInt(byte[] b, int at, int v) { b[at] = (byte)(v >> 24); b[at + 1] = (byte)(v >> 16); b[at + 2] = (byte)(v >> 8); b[at + 3] = (byte)v; }
+
+    static void Chunk(Stream s, string type, byte[] data)
+    {
+        var len = new byte[4]; WriteInt(len, 0, data.Length);
+        s.Write(len);
+        var body = new byte[4 + data.Length];
+        for (var i = 0; i < 4; i++) body[i] = (byte)type[i];
+        data.CopyTo(body, 4);
+        s.Write(body);
+        var crc = new byte[4]; WriteInt(crc, 0, (int)Crc32(body));
+        s.Write(crc);
+    }
+
+    static uint Crc32(byte[] data)
+    {
+        var crc = 0xFFFFFFFFu;
+        foreach (var b in data)
+        {
+            crc ^= b;
+            for (var k = 0; k < 8; k++) crc = (crc & 1) != 0 ? (crc >> 1) ^ 0xEDB88320u : crc >> 1;
+        }
+        return ~crc;
+    }
+}

tests/agent-wiki-ui.Tests/Support/TestApp.cs

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+using Avalonia;
+using Avalonia.Headless;
+using Avalonia.Themes.Fluent;
+using AgentWikiUi.Tests;
+
+[assembly: AvaloniaTestApplication(typeof(TestAppBuilder))]
+
+namespace AgentWikiUi.Tests;
+
+public class TestApp : Application
+{
+    public override void Initialize() => Styles.Add(new FluentTheme());
+}
+
+public static class TestAppBuilder
+{
+    // Skia reale (non il disegno finto): serve per decodificare davvero le immagini
+    public static AppBuilder BuildAvaloniaApp() => AppBuilder.Configure<TestApp>()
+        .UseSkia()
+        .UseHeadless(new AvaloniaHeadlessPlatformOptions { UseHeadlessDrawing = false });
+}