chore(codegen): fix convention violations from plan implementation review

- Use PythonNames.Self constant instead of magic "self" string in
  ResolveExpressionTypeFromSymbols (RoslynEmitter.Expressions.cs)
- Remove redundant _sourceVariableNames.Add in GenerateCatchClauses
  (already collected by CollectSourceVariableNames pre-gen pass)

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

Author: antonsynd

src/Sharpy.Compiler/CodeGen/RoslynEmitter.Expressions.cs

@@ -255,7 +255,7 @@ private ExpressionSyntax GenerateStringLiteral(StringLiteral literal)
         {
             // For self.x, look up on the current type
             if (memberAccess.Object is Identifier obj
-                && string.Equals(obj.Name, "self", StringComparison.OrdinalIgnoreCase)
+                && string.Equals(obj.Name, PythonNames.Self, StringComparison.OrdinalIgnoreCase)
                 && _currentTypeSymbol is not null)
             {
                 var field = _currentTypeSymbol.Fields.Find(f => f.Name == memberAccess.Member);

src/Sharpy.Compiler/CodeGen/RoslynEmitter.Statements.cs

@@ -1827,8 +1827,6 @@ private List<CatchClauseSyntax> GenerateCatchClauses(ImmutableArray<ExceptHandle
                         ? $"{baseName}_{_variableVersions[baseName]}"
                         : baseName;
 
-                    _sourceVariableNames.Add(baseName);
-
                     var catchBlock = Block(handler.Body.SelectMany(GenerateBodyStatements));
                     var declaration = CatchDeclaration(exceptionType, Identifier(exceptionVar));