feat: add RARITY_LADDER, rarity_is_downgrade, and next_rarity helpers (#59)

Closes #59

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

rarity_thresholds.py

@@ -144,3 +144,56 @@ def get_batter_thresholds(season: int) -> BatterRarityThresholds:
         return BATTER_THRESHOLDS_2025
     else:
         return BATTER_THRESHOLDS_2024
+
+
+# Ordered from least to most prestigious. Used for ladder comparisons (T4
+# rarity upgrade, downgrade guard). Do not change the order.
+RARITY_LADDER: list[int] = [
+    5,
+    4,
+    3,
+    2,
+    1,
+    99,
+]  # Common → Bronze → Silver → Gold → Diamond → HoF
+
+
+def rarity_is_downgrade(current_rarity_id: int, new_rarity_id: int) -> bool:
+    """Return True if new_rarity_id is a less prestigious tier than current_rarity_id.
+
+    Uses the RARITY_LADDER ordering. Unknown IDs are treated as position 0
+    (worst), so an unknown current rarity will never trigger a downgrade guard.
+    """
+    try:
+        current_pos = RARITY_LADDER.index(current_rarity_id)
+    except ValueError:
+        return False
+    try:
+        new_pos = RARITY_LADDER.index(new_rarity_id)
+    except ValueError:
+        return False
+    return current_pos > new_pos
+
+
+def next_rarity(current_rarity_id: int) -> int | None:
+    """Return the next more-prestigious rarity ID, or None if already at HoF.
+
+    Uses the RARITY_LADDER ordering. Returns None when current_rarity_id is
+    Hall of Fame (99) — the T4 rarity upgrade is a no-op at the top tier.
+    Returns None for any unrecognised rarity ID.
+
+    Examples:
+        next_rarity(5)  → 4   (Common → Bronze)
+        next_rarity(4)  → 3   (Bronze → Silver)
+        next_rarity(3)  → 2   (Silver → Gold)
+        next_rarity(2)  → 1   (Gold → Diamond)
+        next_rarity(1)  → 99  (Diamond → HoF)
+        next_rarity(99) → None (HoF: already at max)
+    """
+    try:
+        pos = RARITY_LADDER.index(current_rarity_id)
+    except ValueError:
+        return None
+    if pos == len(RARITY_LADDER) - 1:
+        return None  # Already at HoF
+    return RARITY_LADDER[pos + 1]