fix: apply cbrt and cap to runestone formula to prevent AFK windfalls

apps/api/src/services/prestige.ts

@@ -20,6 +20,13 @@ const runestonesPerPrestigeLevel = 10;
 const milestoneInterval = 5;
 const milestoneRunestonesPerInterval = 25;
 
+/*
+ * Hard cap on the base runestone yield (before multipliers) to prevent
+ * extreme AFK accumulation from producing game-breaking runestone counts.
+ * With all upgrades (5.625× max) this caps out at ~281 per prestige.
+ */
+const maxBaseRunestones = 50;
+
 /**
  * Calculates the gold threshold required for the next prestige.
  * Formula: BASE * SCALE_FACTOR^prestigeCount — each prestige makes the next threshold harder.
@@ -107,7 +114,9 @@ interface RunestoneParameters {
 
 /**
  * Calculates how many runestones the player earns from a prestige.
- * Formula: floor(sqrt(totalGoldEarned / threshold)) * RUNESTONES_PER_PRESTIGE_LEVEL * runestoneMultiplier.
+ * Formula: min(floor(cbrt(totalGoldEarned / threshold)) * RUNESTONES_PER_PRESTIGE_LEVEL, MAX_BASE) * multipliers.
+ * Uses cube root for stronger diminishing returns than sqrt, and caps the base before multipliers
+ * to prevent extended AFK sessions from producing runestone windfalls.
  * @param parameters - The parameters for the runestone calculation.
  * @param parameters.totalGoldEarned - The total gold earned in the current run.
  * @param parameters.prestigeCount - The current prestige count.
@@ -123,9 +132,11 @@ const calculateRunestones = (parameters: RunestoneParameters): number => {
     echoRunestoneMultiplier = 1,
   } = parameters;
   const threshold = calculatePrestigeThreshold(prestigeCount);
-  const base
-    = Math.floor(Math.sqrt(totalGoldEarned / threshold))
-    * runestonesPerPrestigeLevel;
+  const base = Math.min(
+    Math.floor(Math.cbrt(totalGoldEarned / threshold))
+    * runestonesPerPrestigeLevel,
+    maxBaseRunestones,
+  );
   const runestoneMult = getCategoryMultiplier(
     purchasedUpgradeIds,
     "runestones",