Initial commit: Complete open-source Decal rebuild

All 5 phases of the open-source Decal rebuild:

Phase 1: 14 decompiled .NET projects (Interop.*, Adapter, FileService, DecalUtil)
Phase 2: 10 native DLLs rewritten as C# COM servers with matching GUIDs
  - DecalDat, DHS, SpellFilter, DecalInput, DecalNet, DecalFilters
  - Decal.Core, DecalControls, DecalRender, D3DService
Phase 3: C++ shims for Inject.DLL (D3D9 hooking) and LauncherHook.DLL
Phase 4: DenAgent WinForms tray application
Phase 5: WiX installer and build script

25 C# projects building with 0 errors.
Native C++ projects require VS 2022 + Windows SDK (x86).

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

Managed/Decal.DenAgent/Program.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Threading;
+using System.Windows.Forms;
+
+namespace Decal.DenAgent
+{
+    static class Program
+    {
+        [STAThread]
+        static void Main()
+        {
+            // Single instance check
+            using var mutex = new Mutex(true, "DecalAgentMutex", out bool createdNew);
+            if (!createdNew)
+            {
+                MessageBox.Show("Decal Agent is already running.", "Decal Agent",
+                    MessageBoxButtons.OK, MessageBoxIcon.Information);
+                return;
+            }
+
+            Application.EnableVisualStyles();
+            Application.SetCompatibleTextRenderingDefault(false);
+            Application.Run(new TrayContext());
+        }
+    }
+}