feat: comprehensive balance pass (#239)

- fix: boss signature chain maintained through fight results (#148)
- fix: militia cost curve smoothed (100g -> 65g) (#145)
- fix: crystal_shard buffed to epic tier (1.65x/1.2x -> 1.9x/1.3x) (#144)
- fix: click_power recipe ceiling raised and z13-18 progression smoothed (#142)
- close: elder_bark_shield, void_fragment_amulet, soul_bound_catalyst already at target values (#143)

packages/types/src/interfaces/api.ts

@@ -170,6 +170,11 @@ interface BossChallengeResponse {
     adventurerId: string;
     killed:       number;
   }>;
+
+  /**
+   * HMAC-SHA256 signature of the updated state for anti-cheat chain continuity.
+   */
+  signature?: string;
 }
 
 type PrestigeRequest = Record<string, never>;