Surface the type-forwarder bit in the flags filter schema

The schema inferer only walks the enum's declared fields, so the
ECMA-335 Forwarder bit (0x00200000, no member in
System.Reflection.TypeAttributes) was invisible to the per-column
flags filter even though the cell tooltip already shows it. An
explicit per-enum extras table appends it as an independent flag,
making forwarders filterable on the ExportedType table.

Assisted-by: Claude:claude-fable-5[1m]:Claude Code

ILSpy.Tests/Metadata/FlagsSchemaInfererTests.cs

@@ -112,6 +112,18 @@ public class FlagsSchemaInfererTests
 		schema.IndependentFlags.Select(f => f.Name).Should().NotContain("LayoutMask");
 	}
 
+	[Test]
+	public void TypeAttributes_Includes_The_Type_Forwarder_Bit()
+	{
+		// 0x00200000 is the ECMA-335 Forwarder bit (II.23.1.15), set on type-forwarder
+		// ExportedType rows. System.Reflection.TypeAttributes has no member for it, so the
+		// enum field walk alone would leave forwarders unfilterable in the flags filter.
+		var schema = FlagsSchemaInferer.For(typeof(TypeAttributes));
+
+		schema.IndependentFlags.Should().ContainSingle(f => f.Name == "IsTypeForwarder")
+			.Which.Bit.Should().Be(0x00200000u);
+	}
+
 	[Test]
 	public void MethodAttributes_Has_Member_Access_Mutex_Plus_Independent_Modifiers()
 	{

ILSpy/Metadata/FlagsSchemaInferer.cs

@@ -36,6 +36,16 @@ namespace ILSpy.Metadata.Filters
 	{
 		static readonly ConcurrentDictionary<Type, FlagsSchema> cache = new();
 
+		/// <summary>
+		/// Bits that exist in ECMA-335 but have no member in the runtime's enum, so the
+		/// field walk cannot surface them. Appended to the inferred independent flags.
+		/// </summary>
+		static readonly Dictionary<Type, IndependentFlag[]> extraFlags = new() {
+			// ECMA-335 II.23.1.15 Forwarder: set on ExportedType rows that forward the
+			// type to another assembly.
+			[typeof(TypeAttributes)] = [new IndependentFlag("IsTypeForwarder", 0x00200000)],
+		};
+
 		public static FlagsSchema For(Type enumType)
 		{
 			ArgumentNullException.ThrowIfNull(enumType);
@@ -90,6 +100,9 @@ namespace ILSpy.Metadata.Filters
 					independentFlags.Add(new IndependentFlag(name, value));
 			}
 
+			if (extraFlags.TryGetValue(enumType, out var extras))
+				independentFlags.AddRange(extras);
+
 			// Build the final mutex groups. Drop empty buckets — a *Mask field whose
 			// values weren't named in the enum (or were aliased away) doesn't earn a
 			// dropdown section. Each group always leads with a synthesised "(none)" entry