fix: add Unity type stubs for DocFX v2 compilation

DocFX v2 uses Roslyn to compile C# files. Without UnityEngine.dll,
types like MonoBehaviour, AudioClip, etc. fail to resolve, causing:
- 14 CS0246 compilation warnings
- Broken <see cref="AudioClip"/> → !:AudioClip (UnityXrefMaps can't link it)
- Missing inheritance chain in generated docs

UnityStubs/UnityEngine.cs provides minimal stubs for the types used in
Player.cs. Placed outside Assets/ so Unity ignores it. The SDK auto-includes
it in DocFxForUnity.csproj. filterConfig.yml excludes the UnityEngine
namespace so stubs never appear as published API pages.

Result: 0 compilation errors, full inheritance chain, correct xref links.

https://claude.ai/code/session_01HDAawqjLMudEktivP5ifXw

Author: claude

UnityStubs/UnityEngine.cs

@@ -0,0 +1,26 @@
+// Minimal Unity type stubs for DocFX documentation generation.
+// Outside Assets/ so Unity ignores this file at build time.
+using System;
+
+namespace UnityEngine
+{
+    public class Object {}
+    public class Component : Object {}
+    public class Behaviour : Component {}
+    public class MonoBehaviour : Behaviour {}
+
+    public class AudioClip : Object {}
+
+    public class AudioSource : Behaviour
+    {
+        public void PlayOneShot(AudioClip clip) {}
+    }
+
+    public static class Debug
+    {
+        public static void Log(object message) {}
+    }
+
+    [AttributeUsage(AttributeTargets.Field)]
+    public sealed class SerializeFieldAttribute : Attribute {}
+}