feat: implement for-loops with proper scope management

- Add support for C-style for loops: for (init; condition; increment) { body }
- Implement three-level scoping: loop context + per-iteration contexts
- Add semicolon disabler for increment statements using RAII guards
- Support break/continue/return control flow with scope validation
- Fix semicolon handling in parser for flexible for-loop syntax

include/context_forward.hpp

@@ -4,6 +4,6 @@
 
 namespace Fig
 {
-    struct Context;
+    class Context;
     using ContextPtr = std::shared_ptr<Context>;
 };