Twig: Arbitrary PHP code execution via `_self.(<string>)` macro-reference compilation

### Description The `obj.(expr)` dynamic-attribute syntax (added in 3.15.0 as the replacement for the deprecated `attribute()` function) lets the attribute be an arbitrary expression. When the receiver is `_self` (or any `{% import %}` alias) and the parenthesised expression is a string literal, `DotExpressionParser` short-circuits to the macro-call path and concatenates the attacker-controlled string into a `MacroReferenceExpression` name with no identifier validation. `MacroReferenceExpression::compile()` then emits that name raw into the generated PHP source. An attacker who can supply template source can inject arbitrary PHP into the compiled template and execute it at template-load time, before `checkSecurity()` is ever called. This is a complete bypass of `SandboxExtension`, including a globally-enabled sandbox with an empty `SecurityPolicy` allowlist. ### Resolution The parser now validates that the dynamic attribute resolves to a valid macro identifier before routing through `MacroReferenceExpression`, and the macro-reference compiler emits the name through a properly escaped path. ### Credits Twig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix.