feat: add difficulty field to prompts and wire into fight pacing

Round 1-2 picks easy prompts, 3-4 medium, 5+ hard (when tagged).
Graceful fallback to any prompt when no difficulty-tagged prompts exist.

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

server/src/engine/challenges.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool } from './challenges.js'
+import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool, roundToDifficulty } from './challenges.js'
 
 describe('pickChallenge', () => {
   it('returns a valid challenge', () => {
@@ -172,6 +172,45 @@ describe('getAnswerPool', () => {
   })
 })
 
+describe('roundToDifficulty', () => {
+  it('rounds 1-2 are easy', () => {
+    expect(roundToDifficulty(1)).toBe('easy')
+    expect(roundToDifficulty(2)).toBe('easy')
+  })
+
+  it('rounds 3-4 are medium', () => {
+    expect(roundToDifficulty(3)).toBe('medium')
+    expect(roundToDifficulty(4)).toBe('medium')
+  })
+
+  it('rounds 5+ are hard', () => {
+    expect(roundToDifficulty(5)).toBe('hard')
+    expect(roundToDifficulty(6)).toBe('hard')
+    expect(roundToDifficulty(10)).toBe('hard')
+  })
+})
+
+describe('pickChallenge with roundNumber', () => {
+  it('accepts roundNumber parameter without error', () => {
+    const c = pickChallenge(new Set(), null, undefined, 1)
+    expect(c).toBeTruthy()
+    expect(c.prompt).toBeTruthy()
+  })
+
+  it('works for all round numbers 1-10', () => {
+    for (let r = 1; r <= 10; r++) {
+      const c = pickChallenge(new Set(), null, undefined, r)
+      expect(c.type).toBeTruthy()
+      expect(c.prompt).toBeTruthy()
+    }
+  })
+
+  it('still works without roundNumber (backward compatible)', () => {
+    const c = pickChallenge(new Set(), null)
+    expect(c).toBeTruthy()
+  })
+})
+
 describe('prompt data integrity', () => {
   it('all 800 prompts are accessible via pickChallenge', () => {
     // Run enough picks to verify the system works