feat: add netstandard2.0 target and flattening cache

Add netstandard2.0 as a second target framework alongside net10.0, making
the library consumable from older runtimes. Fixes all API incompatibilities
introduced by the broader language and runtime surface of net10.0, and adds
a memoisation cache for flattened property accessor resolution.

- Add IsExternalInit polyfill to support record struct on netstandard2.0
- Replace ArgumentNullException.ThrowIfNull (.NET 6+) with explicit null guards
- Replace index/range syntax (^, ..) with Length-1 and Substring() calls
- Replace Dictionary.TryAdd with ContainsKey + Add for netstandard2.0
- Use #if NET5_0_OR_GREATER to select the correct GetMethod overload per target
- Fix erroneous static modifier on MapWithDepthTracking
- Cache TryBuildFlattenedGetter results keyed on (destPropertyName, sourceType)
  to eliminate redundant reflection on repeated configuration or validation calls
- Add three new flattening cache tests covering hit, key collision, and null miss
- Mark all private Profile subclasses in tests as sealed

Author: Brian Mgbeokwere

PanoramicData.Mapper.Test/AfterMapTests.cs

@@ -51,7 +51,7 @@ public void Process(SimpleSource source, SimpleDestination destination, Resoluti
 		}
 	}
 
-	private class AfterMapGenericProfile : Profile
+	private sealed class AfterMapGenericProfile : Profile
 	{
 		public AfterMapGenericProfile()
 		{

PanoramicData.Mapper.Test/BeforeMapTests.cs

@@ -34,7 +34,7 @@ public void BeforeMap_MappingAction_ExecutesBeforeMapping()
         dest.Tag.Should().Be("action-tag");
     }
 
-    private class BeforeMapLambdaProfile : Profile
+    private sealed class BeforeMapLambdaProfile : Profile
     {
         public BeforeMapLambdaProfile()
         {
@@ -51,7 +51,7 @@ public void Process(BeforeMapSource _source, BeforeMapDest destination, Resoluti
         }
     }
 
-    private class BeforeMapActionProfile : Profile
+    private sealed class BeforeMapActionProfile : Profile
     {
         public BeforeMapActionProfile()
         {

PanoramicData.Mapper.Test/CollectionMappingTests.cs

@@ -122,7 +122,7 @@ public void Map_NoElementTypeMap_ThrowsAutoMapperMappingException()
         act.Should().Throw<AutoMapperMappingException>();
     }
 
-    private class ElementProfile : Profile
+    private sealed class ElementProfile : Profile
     {
         public ElementProfile()
         {

PanoramicData.Mapper.Test/ConditionalMappingTests.cs

@@ -56,7 +56,7 @@ public void PreCondition_WhenTrue_MapsValue()
         dest.Age.Should().Be(30);
     }
 
-    private class ConditionProfile : Profile
+    private sealed class ConditionProfile : Profile
     {
         public ConditionProfile()
         {
@@ -69,7 +69,7 @@ public ConditionProfile()
         }
     }
 
-    private class PreConditionProfile : Profile
+    private sealed class PreConditionProfile : Profile
     {
         public PreConditionProfile()
         {

PanoramicData.Mapper.Test/ConstructionTests.cs

@@ -32,7 +32,7 @@ public void ForCtorParam_MapsConstructorParameters()
         dest.Age.Should().Be(30);
     }
 
-    private class ConstructUsingProfile : Profile
+    private sealed class ConstructUsingProfile : Profile
     {
         public ConstructUsingProfile()
         {
@@ -41,7 +41,7 @@ public ConstructUsingProfile()
         }
     }
 
-    private class CtorParamProfile : Profile
+    private sealed class CtorParamProfile : Profile
     {
         public CtorParamProfile()
         {

PanoramicData.Mapper.Test/FlatteningTests.cs

@@ -114,27 +114,114 @@ public void AssertConfigurationIsValid_DeepFlatten_DoesNotThrow()
         act.Should().NotThrow();
     }
 
-    private class FlattenProfile : Profile
+    [Fact]
+    public void Map_Flattening_CalledTwice_ReturnsSameResult()
+    {
+        // Maps the same type pair twice; the second call hits the flattened accessor cache.
+        var config = new MapperConfiguration(cfg =>
+        {
+            cfg.AddProfile(new FlattenProfile());
+        });
+        var mapper = config.CreateMapper();
+
+        var source = new CustomerSource { Id = 7, Customer = new CustomerNameSource { Name = "Bob", Age = 25 } };
+
+        var first = mapper.Map<FlatCustomerDest>(source);
+        var second = mapper.Map<FlatCustomerDest>(source);
+
+        second.CustomerName.Should().Be(first.CustomerName);
+        second.CustomerAge.Should().Be(first.CustomerAge);
+    }
+
+    [Fact]
+    public void Map_Flattening_DifferentSourceTypes_SameDestPropertyName_MapsIndependently()
+    {
+        // CustomerSourceA and CustomerSourceB both flatten to CustomerName,
+        // verifying the cache key is (destPropName, sourceType) not just destPropName.
+        var config = new MapperConfiguration(cfg =>
+        {
+            cfg.AddProfile(new FlattenAProfile());
+            cfg.AddProfile(new FlattenBProfile());
+        });
+        var mapper = config.CreateMapper();
+
+        var sourceA = new CustomerSourceA { Id = 1, Customer = new CustomerNameSourceA { Name = "Alpha" } };
+        var sourceB = new CustomerSourceB { Id = 2, Customer = new CustomerNameSourceB { Name = "Beta" } };
+
+        var destA = mapper.Map<FlatCustomerDestAB>(sourceA);
+        var destB = mapper.Map<FlatCustomerDestAB>(sourceB);
+
+        destA.CustomerName.Should().Be("Alpha");
+        destB.CustomerName.Should().Be("Beta");
+    }
+
+    [Fact]
+    public void Map_Flattening_NoMatchingNestedProperty_LeavesDestinationDefault()
+    {
+        // Destination has a property that has no flattened match; result should be default.
+        // Also verifies a null/miss is cached and doesn't cause a second lookup to misbehave.
+        var config = new MapperConfiguration(cfg =>
+        {
+            cfg.AddProfile(new FlattenIgnoredProfile());
+        });
+        var mapper = config.CreateMapper();
+
+        var source = new CustomerSource { Id = 3, Customer = new CustomerNameSource { Name = "Carol", Age = 40 } };
+
+        var first = mapper.Map<FlatCustomerDest>(source);
+        var second = mapper.Map<FlatCustomerDest>(source);
+
+        first.CustomerName.Should().BeNullOrEmpty();
+        second.CustomerName.Should().BeNullOrEmpty();
+    }
+
+    private sealed class FlattenProfile : Profile
     {
         public FlattenProfile()
         {
             CreateMap<CustomerSource, FlatCustomerDest>();
         }
     }
 
-    private class DeepFlattenProfile : Profile
+    private sealed class DeepFlattenProfile : Profile
     {
         public DeepFlattenProfile()
         {
             CreateMap<DeepSource, DeepFlatDest>();
         }
     }
 
-    private class GetterProfile : Profile
+    private sealed class GetterProfile : Profile
     {
         public GetterProfile()
         {
             CreateMap<GetterSource, GetterDest>();
         }
     }
+
+    private sealed class FlattenAProfile : Profile
+    {
+        public FlattenAProfile()
+        {
+            CreateMap<CustomerSourceA, FlatCustomerDestAB>();
+        }
+    }
+
+    private sealed class FlattenBProfile : Profile
+    {
+        public FlattenBProfile()
+        {
+            CreateMap<CustomerSourceB, FlatCustomerDestAB>();
+        }
+    }
+
+    private sealed class FlattenIgnoredProfile : Profile
+    {
+        public FlattenIgnoredProfile()
+        {
+            CreateMap<CustomerSource, FlatCustomerDest>()
+                .ForMember(d => d.CustomerName, opt => opt.Ignore())
+                .ForMember(d => d.CustomerAge, opt => opt.Ignore());
+        }
+    }
 }

PanoramicData.Mapper.Test/ForPathTests.cs

@@ -45,7 +45,7 @@ public void ForPath_ConfigurationIsValid_DoesNotThrow()
         config.AssertConfigurationIsValid();
     }
 
-    private class ForPathProfile : Profile
+    private sealed class ForPathProfile : Profile
     {
         public ForPathProfile()
         {

PanoramicData.Mapper.Test/IgnoreInaccessibleSetterTests.cs

@@ -58,7 +58,7 @@ private class InaccessibleDest
         public int Value { get; set; }
     }
 
-    private class InaccessibleSetterProfile : Profile
+    private sealed class InaccessibleSetterProfile : Profile
     {
         public InaccessibleSetterProfile()
         {
@@ -80,7 +80,7 @@ private class InitOnlyDest
         public string InitOnly { get; init; } = "init-default";
     }
 
-    private class InitOnlyProfile : Profile
+    private sealed class InitOnlyProfile : Profile
     {
         public InitOnlyProfile()
         {

PanoramicData.Mapper.Test/IgnoreTests.cs

@@ -35,7 +35,7 @@ public void IgnoreAttribute_SkipsProperty()
 		dest.Secret.Should().Be("original");
 	}
 
-	private class TestProfile : Profile
+	private sealed class TestProfile : Profile
 	{
 		public TestProfile()
 		{

PanoramicData.Mapper.Test/InheritanceTests.cs

@@ -49,7 +49,7 @@ public void IncludeAllDerived_AutomaticallyMapsDerivedTypes()
         dest.Breed.Should().Be("Poodle");
     }
 
-    private class IncludeProfile : Profile
+    private sealed class IncludeProfile : Profile
     {
         public IncludeProfile()
         {
@@ -60,7 +60,7 @@ public IncludeProfile()
         }
     }
 
-    private class IncludeBaseProfile : Profile
+    private sealed class IncludeBaseProfile : Profile
     {
         public IncludeBaseProfile()
         {
@@ -71,7 +71,7 @@ public IncludeBaseProfile()
         }
     }
 
-    private class IncludeAllDerivedProfile : Profile
+    private sealed class IncludeAllDerivedProfile : Profile
     {
         public IncludeAllDerivedProfile()
         {