fix(core): add Slice.GetSlice overload for Bytes and update skip reasons

- Add GetSlice(Bytes, int?, int?, int?) to Slice.cs, delegating to bytes.Slice()
- Un-skip bytes_slicing fixture (now passes)
- Update stdlib_collections_defaultdict.skip with issue references (#553-555)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonsynd

src/Sharpy.Compiler.Tests/Integration/TestFixtures/bytes_slicing.skip

@@ -0,0 +1 @@
+DefaultDict codegen issues: #553 (factory function), #554 (generic type args), #555 (missing key default)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/stdlib_collections_defaultdict.skip

@@ -150,5 +150,14 @@ public static string GetSlice(string str, int? start, int? end, int? step)
                 return sb.ToString();
             }
         }
+
+        /// <summary>
+        /// Slice a Bytes using Python slice semantics.
+        /// Used by generated code for bytes[start:stop:step] expressions.
+        /// </summary>
+        public static Bytes GetSlice(Bytes bytes, int? start, int? end, int? step)
+        {
+            return bytes.Slice(start, end, step);
+        }
     }
 }