Replace tiers with achievements

src/progress.rs

@@ -4,13 +4,16 @@ use anyhow::Result;
 use serde::{Deserialize, Serialize};
 use std::path::PathBuf;
 
-use crate::levels::Difficulty;
+use crate::levels::Nugget;
 
 #[derive(Default, Serialize, Deserialize)]
+#[serde(default)]
 pub struct Progress {
-    pub tier: Option<Difficulty>,
-    pub completed: Vec<u8>,
-    pub current_level: u8,
+    /// Nuggets awarded per level: `(level_id, nugget)` in order of
+    /// completion. A level appears at most once (re-runs after reset
+    /// rebuild the list from scratch).
+    pub nuggets: Vec<(u8, Nugget)>,
+    pub current_level: u8, // 1-indexed; 0 means a new game
     pub current_seed: u64,
     pub attempts: u32,
 }