feat: scale adventurer tier costs progressively

Each adventurer tier now has a unique baseCost that scales ~7x per
tier (L1: 10 → L32: ~2.5e27). The per-count scaling factor (1.15×)
is unchanged.

Closes #10

packages/types/src/interfaces/Adventurer.ts

@@ -11,6 +11,8 @@ export interface Adventurer {
   name: string;
   class: AdventurerClass;
   level: number;
+  /** Base cost for the first purchase of this tier (scales by 1.15× per count) */
+  baseCost: number;
   /** Base gold generated per second */
   goldPerSecond: number;
   /** Base essence generated per second */