Add TreatWarningsAsErrors -> True for release builds, cleanup

Directory.Build.props

@@ -0,0 +1,5 @@
+<Project>
+    <PropertyGroup Condition="'$(Configuration)' == 'Release'">
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    </PropertyGroup>
+</Project>

external/Directory.Build.props

@@ -0,0 +1,6 @@
+<Project>
+    <!--
+        This is intentionally blank, so any changes locally don't get applied to
+        any external projects that reference this one.
+    -->
+</Project>

src/InteropGen/Generator.cs

@@ -67,7 +67,7 @@ public static void Generate(NativeModel model, CodeGen gen)
             }
 
 
-            bool TryMapCommonType(NativeType type, bool allowStrings, out string res)
+            bool TryMapCommonType(NativeType type, bool allowStrings, [System.Diagnostics.CodeAnalysis.MaybeNullWhen(false)] out string res)
             {
                 string? Map()
                 {
@@ -296,6 +296,11 @@ string MapDotnetType(NativeType type, bool allowHandles)
 
                                 void GenerateHandleReturn(NativeHandle nativeHandleType)
                                 {
+                                    // Ownership follows the "New" naming rule, NOT the method/factory
+                                    // classification: a New-suffixed function whose first parameter is a
+                                    // handle is modeled as an instance method (isFactory == false) but still
+                                    // returns a freshly-created, caller-owned handle. Using isFactory here
+                                    // would emit RetainFromNative for those and over-retain (leak).
                                     if (f.Name.EndsWith("New"))
                                         gen.Line($"return new {nativeHandleType.Name}(ret);");
                                     else
@@ -308,7 +313,13 @@ void GenerateHandleReturn(NativeHandle nativeHandleType)
                                         GenerateHandleReturn(nativeHandle);
                                     else if(f.ReturnType is NativeNullableType { ElementType: NativeHandle nativeHandleType})
                                     {
-                                        gen.Line("if(ret == null) return null;");
+                                        // Mirror GenerateHandleReturn's New-based branch: New-suffixed
+                                        // functions return a SafeHandle from P/Invoke (compare to null);
+                                        // other handle-returning functions return a raw IntPtr (compare to IntPtr.Zero).
+                                        if (f.Name.EndsWith("New"))
+                                            gen.Line("if(ret == null) return null;");
+                                        else
+                                            gen.Line("if(ret == global::System.IntPtr.Zero) return null;");
                                         GenerateHandleReturn(nativeHandleType);
                                     }
                                     else

src/InteropGen/NativeModel.cs

@@ -82,7 +82,7 @@ class NativeEnum(string name) : NativeType
 
 class NativeHandle : NativeType
 {
-    public string Name { get; set; }
+    public required string Name { get; set; }
     public List<NativeFunction> Methods { get; set; } = new();
     public List<NativeFunction> Factories { get; set; } = new();
     public override string ToString() => Name;
@@ -95,7 +95,7 @@ class ExternalNativeType(string name) : NativeType
 
 class NativeFunctionPointer : NativeType
 {
-    public NativeType ReturnType { get; set; }
+    public required NativeType ReturnType { get; set; }
     public List<NativeVar> Parameters { get; set; } = new();
 }
 
@@ -108,9 +108,9 @@ class NativeNamedFunctionPointer(string name) : NativeFunctionPointer
 
 class NativeFunction
 {
-    public string Name { get; set; }
-    public NativeType ReturnType { get; set; }
-    public List<NativeVar> Parameters { get; set; }
+    public required string Name { get; set; }
+    public required NativeType ReturnType { get; set; }
+    public required List<NativeVar> Parameters { get; set; }
 }
 
 class NativeModel