task 2.1-2.2: implement MarshalCache<T> with tests

Add MarshalCache<T> static class that computes Size, SizeU,
TypeRequiresMarshal, IsIntPtr, TypeCode, and RealType once per type
in the static constructor. Handles bool (size=1), enums (underlying
type), and MarshalAs-attributed fields (TypeRequiresMarshal).

12 new tests covering: blittable sizes, bool size, enum size, struct
size, marshal-required flag, IsIntPtr, computed-once caching.
All passing.

WhiteMagicTest/MarshalCacheTests.cs

@@ -0,0 +1,111 @@
+using System.Runtime.InteropServices;
+using WhiteMagic;
+
+namespace WhiteMagicTest;
+
+/// <summary>
+/// Tests for <see cref="MarshalCache{T}"/>: blittable size, marshal-required flag,
+/// IsIntPtr, and computed-once behavior.
+/// </summary>
+public class MarshalCacheTests
+{
+    [Fact]
+    public void Size_for_int_is_4()
+    {
+        Assert.Equal(4, MarshalCache<int>.Size);
+    }
+
+    [Fact]
+    public void Size_for_byte_is_1()
+    {
+        Assert.Equal(1, MarshalCache<byte>.Size);
+    }
+
+    [Fact]
+    public void Size_for_IntPtr_matches_native_pointer_size()
+    {
+        Assert.Equal(IntPtr.Size, MarshalCache<IntPtr>.Size);
+    }
+
+    [Fact]
+    public void Size_for_bool_is_1()
+    {
+        Assert.Equal(1, MarshalCache<bool>.Size);
+    }
+
+    [Fact]
+    public void Size_for_enum_matches_underlying_type()
+    {
+        Assert.Equal(4, MarshalCache<DayOfWeek>.Size);
+    }
+
+    [Fact]
+    public void Size_for_blittable_struct_is_accurate()
+    {
+        Assert.Equal(8, MarshalCache<BlittableStruct>.Size);
+    }
+
+    [Fact]
+    public void TypeRequiresMarshal_is_false_for_blittable_types()
+    {
+        Assert.False(MarshalCache<int>.TypeRequiresMarshal);
+        Assert.False(MarshalCache<long>.TypeRequiresMarshal);
+        Assert.False(MarshalCache<BlittableStruct>.TypeRequiresMarshal);
+    }
+
+    [Fact]
+    public void TypeRequiresMarshal_is_true_for_types_with_MarshalAs_field()
+    {
+        Assert.True(MarshalCache<MarshalAsStruct>.TypeRequiresMarshal);
+    }
+
+    [Fact]
+    public void IsIntPtr_is_true_for_IntPtr()
+    {
+        Assert.True(MarshalCache<IntPtr>.IsIntPtr);
+    }
+
+    [Fact]
+    public void IsIntPtr_is_false_for_non_IntPtr_types()
+    {
+        Assert.False(MarshalCache<int>.IsIntPtr);
+        Assert.False(MarshalCache<long>.IsIntPtr);
+        Assert.False(MarshalCache<BlittableStruct>.IsIntPtr);
+    }
+
+    [Fact]
+    public void All_properties_are_computed_once_and_cached()
+    {
+        int size1 = MarshalCache<int>.Size;
+        bool marshal1 = MarshalCache<int>.TypeRequiresMarshal;
+        bool intPtr1 = MarshalCache<int>.IsIntPtr;
+
+        int size2 = MarshalCache<int>.Size;
+        bool marshal2 = MarshalCache<int>.TypeRequiresMarshal;
+        bool intPtr2 = MarshalCache<int>.IsIntPtr;
+
+        Assert.Equal(size1, size2);
+        Assert.Equal(marshal1, marshal2);
+        Assert.Equal(intPtr1, intPtr2);
+    }
+
+    [Fact]
+    public void SizeU_matches_Size_as_uint()
+    {
+        Assert.Equal((uint)MarshalCache<int>.Size, MarshalCache<int>.SizeU);
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct BlittableStruct
+    {
+        public int X;
+        public int Y;
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct MarshalAsStruct
+    {
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
+        public byte[] Data;
+    }
+}