feat: comprehensive balance and bug fix pass (#240)

## Summary

- **fix(#148)**: Boss fights now return a fresh HMAC signature in the response; both the manual and auto-boss paths update `signatureReference` from it, ending the signature-mismatch loop that stopped auto-boss after the first fight
- **fix(#145)**: Militia `baseCost` lowered from 100g → 65g, smoothing the peasant→militia jump from 10× to ~6.5×
- **fix(#144)**: `crystal_shard` buffed from `1.65×/1.2×` → `1.9×/1.3×` — now competitive as an epic trinket
- **fix(#142)**: Click-power recipe progression smoothed across zones 13–18 and ceiling raised: z13 1.20→1.22, z15 1.22→1.25, z17 1.25→1.28, z18 1.28→1.30
- **close(#143)**: `elder_bark_shield` (1.2×), `void_fragment_amulet` (1.15×), and `soul_bound_catalyst` (1.2×) are all already at or above their target values from a prior pass

Closes #148
Closes #145
Closes #144
Closes #142

Reviewed-on: #240
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/api/src/data/adventurers.ts

@@ -21,7 +21,7 @@ export const defaultAdventurers: Array<Adventurer> = [
     unlocked:         true,
   },
   {
-    baseCost:         100,
+    baseCost:         65,
     class:            "warrior",
     combatPower:      3,
     count:            0,