Replace tiers with achievements

src/levels/mod.rs

@@ -15,37 +15,47 @@ pub mod l02_kv;
 
 use serde::{Deserialize, Serialize};
 
-/// Game-wide difficulty. Chosen once on the TierSelect screen; persisted
-/// in `Progress.tier`. Maps to a fixed passing threshold per level.
+/// Awarded per level based on the grade score. Replaces the old
+/// game-wide difficulty tier. Thresholds:
+/// - Gold   ≥ 95 %
+/// - Silver ≥ 80 %
+/// - Bronze ≥ 70 %
+/// - Below 70 % → no nugget (retry).
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
-pub enum Difficulty {
-    Easy,
-    Medium,
-    Hard,
+pub enum Nugget {
+    Bronze,
+    Silver,
+    Gold,
 }
 
-impl Difficulty {
-    pub fn threshold(self) -> f64 {
-        match self {
-            Self::Easy => 0.70,
-            Self::Medium => 0.80,
-            Self::Hard => 0.95,
+impl Nugget {
+    /// Map a grade ratio (0.0..=1.0) to a nugget. `None` means the
+    /// player didn't clear the level — retry without advancing.
+    pub fn from_score(score: f64) -> Option<Self> {
+        if score >= 0.95 {
+            Some(Nugget::Gold)
+        } else if score >= 0.80 {
+            Some(Nugget::Silver)
+        } else if score >= 0.70 {
+            Some(Nugget::Bronze)
+        } else {
+            None
         }
     }
 
-    pub fn label(self) -> &'static str {
+    pub fn emoji(self) -> &'static str {
         match self {
-            Self::Easy => "Easy (70%)",
-            Self::Medium => "Medium (80%)",
-            Self::Hard => "Hard (95%)",
+            Self::Bronze => "🥉",
+            Self::Silver => "🥈",
+            Self::Gold => "🥇",
         }
     }
 
     pub fn name(self) -> &'static str {
         match self {
-            Self::Easy => "Easy",
-            Self::Medium => "Medium",
-            Self::Hard => "Hard",
+            Self::Bronze => "Bronze",
+            Self::Silver => "Silver",
+            Self::Gold => "Gold",
         }
     }
 }