Annotate intentional analyzer-violations with [SuppressMessage].

Four cases where the analyzer rule conflicts with intentional design:

* EmptyList<T>.IDisposable.Dispose (CA1063) — explicit IDisposable on
  IEnumerator<T>; making it public would conflict with the rest of the
  IList<T> / IEnumerator<T> surface.
* MetadataFile.SectionHeaders (CA1065) — throw documents that this
  MetadataFileKind has no PE sections; PE-like derived kinds override.
* LongSet.GetHashCode + LongSet itself (CA1065 + CA2231) — explicit
  guards against using LongSet in hash containers / via equality
  operators; SetEquals is the supported comparison and
  IEquatable<LongSet>.Equals is itself [Obsolete].
* AnnotationList.Clone (CA2002) — AnnotationList is a private nested
  type; the surrounding Annotatable class deliberately locks on the
  AnnotationList instance to serialize annotation reads/writes, and
  external code cannot obtain a reference to it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ICSharpCode.Decompiler/CSharp/Syntax/IAnnotatable.cs

@@ -18,6 +18,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 
@@ -115,6 +116,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 			}
 
+			[SuppressMessage("Reliability", "CA2002:Do not lock on objects with weak identity",
+				Justification = "AnnotationList is a private nested type — the surrounding Annotatable class deliberately locks on the AnnotationList instance to serialize annotation reads/writes; external code cannot obtain a reference to it.")]
 			public object Clone()
 			{
 				lock (this)

ICSharpCode.Decompiler/Metadata/MetadataFile.cs

@@ -22,6 +22,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection.Metadata;
 using System.Reflection.PortableExecutable;
@@ -285,6 +286,8 @@ namespace ICSharpCode.Decompiler.Metadata
 			throw new BadImageFormatException("This metadata file does not support sections.");
 		}
 
+		[SuppressMessage("Design", "CA1065:Do not raise exceptions in unexpected locations",
+			Justification = "Throw signals that this MetadataFileKind has no PE sections; derived PE-like kinds override.")]
 		public virtual ImmutableArray<SectionHeader> SectionHeaders => throw new BadImageFormatException("This metadata file does not support sections.");
 
 		/// <summary>

ICSharpCode.Decompiler/Util/EmptyList.cs

@@ -20,6 +20,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace ICSharpCode.Decompiler.Util
 {
@@ -99,6 +100,8 @@ namespace ICSharpCode.Decompiler.Util
 			get { throw new NotSupportedException(); }
 		}
 
+		[SuppressMessage("Usage", "CA1063:Implement IDisposable Correctly",
+			Justification = "Explicit IDisposable implementation for IEnumerator<T>; intentional no-op for the singleton.")]
 		void IDisposable.Dispose()
 		{
 		}

ICSharpCode.Decompiler/Util/LongSet.cs

@@ -21,6 +21,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 namespace ICSharpCode.Decompiler.Util
@@ -28,6 +29,8 @@ namespace ICSharpCode.Decompiler.Util
 	/// <summary>
 	/// An immutable set of longs, that is implemented as a list of intervals.
 	/// </summary>
+	[SuppressMessage("Usage", "CA2231:Overload operator equals on overriding value type Equals",
+		Justification = "Equality on LongSet is intentionally only available via SetEquals — the IEquatable<LongSet>.Equals overload is itself [Obsolete] in favor of SetEquals.")]
 	public struct LongSet : IEquatable<LongSet>
 	{
 		/// <summary>
@@ -362,6 +365,8 @@ namespace ICSharpCode.Decompiler.Util
 			return obj is LongSet && SetEquals((LongSet)obj);
 		}
 
+		[SuppressMessage("Design", "CA1065:Do not raise exceptions in unexpected locations",
+			Justification = "Throw is an explicit guard against using LongSet in hash-based containers; use SetEquals for comparison.")]
 		public override int GetHashCode()
 		{
 			throw new NotImplementedException();