fix(semantic): add bytes integer indexing type inference

Bytes indexing (b[0]) was returning UnknownType because the __getitem__
protocol stub lacks return type information. Add explicit bytes case in
InferIndexAccessType (matching the existing str pattern) to return int,
consistent with Python's bytes[int] → int behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonsynd

src/Sharpy.Compiler/Semantic/TypeInferenceService.cs

@@ -779,6 +779,12 @@ c.Parameters[1].Type is GenericType paramGeneric &&
             return SemanticType.Str;
         }
 
+        // Bytes - integer indexing returns int (b[0] → 104)
+        if (container is UserDefinedType { Name: BuiltinNames.Bytes })
+        {
+            return SemanticType.Int;
+        }
+
         // User-defined types with __getitem__
         TypeSymbol? typeSymbol = container switch
         {