fix: Allow arithmetic on unknown types in type checker

- Modified type checker to not error on arithmetic operations with TYPE_UNKNOWN operands
- This allows dynamic typing in function parameters (e.g., fib benchmark)
- Replaces errors with TYPE_UNKNOWN propagation for mixed/unknown type arithmetic
- Fixes Windows CI benchmark failures due to overly strict type checking

Author: ProgrammerKR

src/compiler/type_checker.c

@@ -178,11 +178,11 @@ static TypeInfo checkBinary(TypeChecker* checker, Expr* expr) {
                 // Maybe allow String + Int -> String?
                 result = createType(TYPE_STRING);
             } else {
-                error(checker, expr->line, "Type mismatch in binary operation.");
+                // Allow arithmetic on unknown types (for dynamic typing)
                 result = createType(TYPE_UNKNOWN);
             }
         } else {
-            error(checker, expr->line, "Type mismatch in binary operation.");
+            // Allow arithmetic on unknown/mixed types (dynamic typing)
             result = createType(TYPE_UNKNOWN);
         }