balance: smooth prestige income cliff, quadratic milestones, exponential combat scaling (#170, #171)

Reduce income_10 cost 30k→22.5k and income_11 80k→60k (25% cut each)
to ease the late-prestige runestone cliff without collapsing the timeline.

Change prestige milestone bonus from linear (n×25) to quadratic (n²×25)
so high-prestige milestones feel meaningful (P100 = 10k stones).

Replace linear prestige combat multiplier (1 + count×0.1) with exponential
(4^count) in both the tick engine and server-side boss route. Without this
the final boss (2×10^145 HP) was unreachable by ~112 orders of magnitude;
base-4 makes it achievable around P190, consistent with the 6-month target.

apps/web/src/engine/tick.ts

@@ -84,6 +84,12 @@ const checkAchievements = (state: GameState): Array<Achievement> => {
   });
 };
 
+/**
+ * Exponential base for the prestige combat multiplier: Math.pow(base, prestigeCount).
+ * Replaces the former linear formula (1 + count * 0.1) to enable late-game zone progression.
+ */
+export const PRESTIGE_COMBAT_BASE = 4;
+
 /**
  * Maximum value any resource can accumulate to. Beyond this JS floats lose all useful precision.
  */
@@ -296,8 +302,7 @@ export const computeEffectiveAdventurerStats = (
 
   const runestonesIncome = state.prestige.runestonesIncomeMultiplier ?? 1;
   const runestonesEssence = state.prestige.runestonesEssenceMultiplier ?? 1;
-  // eslint-disable-next-line stylistic/no-mixed-operators -- prestige count * factor is clear
-  const prestigeCombatMultiplier = 1 + state.prestige.count * 0.1;
+  const prestigeCombatMultiplier = Math.pow(PRESTIGE_COMBAT_BASE, state.prestige.count);
   const echoIncome = state.transcendence?.echoIncomeMultiplier ?? 1;
   const echoCombatMultiplier = state.transcendence?.echoCombatMultiplier ?? 1;
   const craftedGoldMultiplier
@@ -378,8 +383,7 @@ export const computePartyCombatPower = (state: GameState): number => {
     }
   }
 
-  // eslint-disable-next-line stylistic/no-mixed-operators -- prestige count * factor is clear
-  const prestigeMultiplier = 1 + state.prestige.count * 0.1;
+  const prestigeMultiplier = Math.pow(PRESTIGE_COMBAT_BASE, state.prestige.count);
 
   const equipmentCombatMultiplier = state.equipment.
     filter((item) => {