test: update arena count assertion from 25 to 40

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/arenas.test.ts

@@ -2,8 +2,8 @@ import { describe, it, expect } from 'vitest'
 import { ARENAS, pickArena, randomArena } from './arenas.js'
 
 describe('ARENAS', () => {
-  it('has 25 arenas', () => {
-    expect(ARENAS.length).toBe(25)
+  it('has 40 arenas', () => {
+    expect(ARENAS.length).toBe(40)
   })
 
   it('all arenas have required fields', () => {