Replace tiers with achievements

src/lib.rs

@@ -69,15 +69,14 @@ mod smoke {
         // (4) Progress round-trips through the same YAML pipeline that disk
         // save/load will use.
         let p = progress::Progress {
-            tier: Some(levels::Difficulty::Medium),
-            completed: vec![1, 2],
+            nuggets: vec![(1, levels::Nugget::Silver), (2, levels::Nugget::Gold)],
             current_level: 3,
             current_seed: 0xCAFE,
             attempts: 1,
         };
         let s = serde_yaml::to_string(&p).unwrap();
         let loaded: progress::Progress = serde_yaml::from_str(&s).unwrap();
-        assert_eq!(loaded.tier, p.tier);
+        assert_eq!(loaded.nuggets, p.nuggets);
         assert_eq!(loaded.current_level, p.current_level);
         assert_eq!(loaded.current_seed, p.current_seed);
     }