docs: restore Byleth early-level growth reference

Author: jinghaihan

README.md

@@ -86,10 +86,25 @@ vanilla random system, but it does not undo stats already earned.
 
 ## Early-game test reference
 
-No unit should get an empty or single-stat level-up while at least two of its
-stats can still increase. If all effective growth rates and counters are zero,
-the fixed tie order grants Strength and Magic. A fallback gain leaves that
-stat's fractional counter unchanged, so it can also grow naturally later.
+For a new game with this plugin active from level 1, these are Byleth's
+expected *per-level increases*. Keep Byleth as a Commoner through level 5;
+do not equip growth-modifying abilities or reach a stat cap. Gain each level
+separately. Compare the stat changes, not Byleth's total stats. A class change
+or a save first played without the plugin will produce different results.
+
+| Level | Expected gains |
+| --- | --- |
+| 1 → 2 | Str, Dex |
+| 2 → 3 | HP, Str, Mag, Dex, Spd, Lck, Def, Cha |
+| 3 → 4 | Str, Res |
+| 4 → 5 | HP, Str, Dex, Spd, Lck, Cha |
+
+Each listed stat gains **one point**; unlisted stats do not change. Byleth's
+1 → 2 gains (Str, Dex) and 3 → 4 gains (Str, Res) include the two-stat
+minimum: those levels would otherwise have zero or only one natural gain.
+No unit should have an empty or single-stat level-up while at least two stats
+can still increase. These are predictions from the plugin's current algorithm,
+not yet hardware-verified results.
 
 ## Documentation
 

src/level_up.rs

@@ -98,6 +98,41 @@ mod tests {
     }
   }
 
+  #[test]
+  fn matches_byleths_early_game_readme_reference() {
+    // HP, Str, Mag, Dex, Spd, Lck, Def, Res, Mov, Cha.
+    let sources = GrowthSources {
+      personal: [45, 45, 35, 45, 45, 45, 35, 30, 0, 45],
+      class: [0; STAT_COUNT], // Commoner.
+      ability_bonus: 0,
+      caps: [99; STAT_COUNT],
+    };
+    let expected = [
+      [0, 1, 0, 1, 0, 0, 0, 0, 0, 0],
+      [1, 1, 1, 1, 1, 1, 1, 0, 0, 1],
+      [0, 1, 0, 0, 0, 0, 0, 1, 0, 0],
+      [1, 1, 0, 1, 1, 1, 0, 0, 0, 1],
+    ];
+    let mut stats = [10; STAT_COUNT];
+    let mut persisted = None;
+
+    for (level, expected_gains) in (1..=4).zip(expected) {
+      let LevelUpDecision::Apply { result, state } =
+        decide_level_up(level, level + 1, stats, sources, persisted)
+      else {
+        panic!("expected Byleth's level-up result");
+      };
+      assert_eq!(
+        result.gains,
+        expected_gains,
+        "level {level} → {}",
+        level + 1
+      );
+      stats = result.stats;
+      persisted = Some(state);
+    }
+  }
+
   #[test]
   fn initializes_with_personal_growth_before_the_first_level() {
     let decision = decide_level_up(1, 2, [10; STAT_COUNT], sources(35, 10, 0), None);