test: add fight length and KO rate distribution test

Simulates 1000 full fights with HP tracking (200 HP start, 7-10 rounds)
to verify average fight length (5-10 rounds) and KO rate (20-80%).
Validates the balance between damage output and HP pool.

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

server/src/engine/lifecycle.test.ts

@@ -420,6 +420,63 @@ describe('fight lifecycle', () => {
     }
   })
 
+  it('average fight lasts 5-10 rounds with ~30-70% KO rate', () => {
+    const STARTING_HP = 200
+    let totalRounds = 0
+    let totalKOs = 0
+    const totalFights = 1000
+
+    for (let f = 0; f < totalFights; f++) {
+      const eloA = 1000 + Math.floor(Math.random() * 800)
+      const eloB = 1000 + Math.floor(Math.random() * 800)
+      const botA = { id: 'fl-a', name: 'A' }
+      const botB = { id: 'fl-b', name: 'B' }
+      const usedTypes = new Set<string>()
+      let hpA = STARTING_HP
+      let hpB = STARTING_HP
+      let comboA = 0
+      let comboB = 0
+      const maxRounds = 7 + Math.floor(Math.random() * 4) // 7-10 rounds like mock.ts
+      let rounds = 0
+
+      for (let r = 0; r < maxRounds; r++) {
+        const challenge = pickChallenge(usedTypes, null, undefined, r + 1)
+        usedTypes.add(challenge.type)
+        const respA = mockResponse(challenge, 'confident', eloA)
+        const respB = mockResponse(challenge, 'confident', eloB)
+        const result = scoreRound(
+          challenge, botA, botB,
+          { answer: respA.answer, timeMs: respA.timeMs, timedOut: respA.timedOut, error: respA.error },
+          { answer: respB.answer, timeMs: respB.timeMs, timedOut: respB.timedOut, error: respB.error },
+          null, comboA, comboB,
+        )
+
+        hpB = Math.max(0, hpB - result.botADamage)
+        hpA = Math.max(0, hpA - result.botBDamage)
+        if (result.winnerId === botA.id) { comboA++; comboB = 0 }
+        else if (result.winnerId === botB.id) { comboB++; comboA = 0 }
+        rounds++
+
+        if (hpA <= 0 || hpB <= 0) {
+          totalKOs++
+          break
+        }
+      }
+      totalRounds += rounds
+    }
+
+    const avgRounds = totalRounds / totalFights
+    const koRate = totalKOs / totalFights
+
+    // Average fight should be 5-10 rounds
+    expect(avgRounds).toBeGreaterThan(4)
+    expect(avgRounds).toBeLessThan(10)
+
+    // KO rate should be reasonable (30-70% of fights end in KO)
+    expect(koRate).toBeGreaterThan(0.2)
+    expect(koRate).toBeLessThan(0.8)
+  })
+
   it('Elo difference correlates with win rate', () => {
     const scenarios = [
       { eloA: 1400, eloB: 1400, expectedAWinMin: 0.40, expectedAWinMax: 0.60 },