Add Solver trait and move common functionality out of concrete solvers

src/main.rs

@@ -1,8 +1,10 @@
-mod solvers;
 mod common;
+mod solvers;
 
 fn main() {
     println!("### Advent of Code 2025 ###\n");
-    solvers::printing_department::PrintingDepartment::new().run();
-    solvers::cafeteria::Cafeteria::new().run();
+    solvers::run(Box::new(
+        solvers::printing_department::PrintingDepartment::new(),
+    ));
+    solvers::run(Box::new(solvers::cafeteria::Cafeteria::new()));
 }