feat: add File.copy and propagate effectful callback effects (WISH-7, WISH-14)

File.copy(source, dest) copies files via interpreter (std::fs::copy) and
C backend (fread/fwrite). Effectful callbacks passed to higher-order
functions like List.map/forEach now propagate their effects to the
enclosing function's inferred effect set.

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

src/types.rs

@@ -956,6 +956,14 @@ impl TypeEnv {
                         params: vec![("path".to_string(), Type::String)],
                         return_type: Type::Unit,
                     },
+                    EffectOpDef {
+                        name: "copy".to_string(),
+                        params: vec![
+                            ("source".to_string(), Type::String),
+                            ("dest".to_string(), Type::String),
+                        ],
+                        return_type: Type::Unit,
+                    },
                 ],
             },
         );