feat: add answer confidence differential as scoring factor

When both bots are correct, the one with higher checkAnswer confidence
(exact match 1.0 vs fuzzy match 0.8) gets up to +1.0 bonus points.
This rewards precise answers over approximate ones, adding another
competitive dimension beyond pure speed.

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

server/src/engine/scoring.test.ts

@@ -218,6 +218,25 @@ describe('scoreRound', () => {
     expect(typeof result.narration).toBe('string')
     expect(result.narration.length).toBeGreaterThan(10)
   })
+
+  it('answer confidence gives bonus — exact match (1.0) beats fuzzy match (0.8)', () => {
+    // Use answers where one answer is a substring of another
+    // "satoshi" against ["satoshi nakamoto"] scores 0.8 (containment)
+    // "satoshi nakamoto" against ["satoshi nakamoto"] scores 1.0 (exact)
+    const challenge = makeChallenge({ answers: ['satoshi nakamoto'] })
+
+    // Both answer at equal speed, but A has exact match, B has fuzzy
+    const result = scoreRound(
+      challenge, botA, botB,
+      makeResponse('satoshi nakamoto', 500),  // exact → 1.0
+      makeResponse('nakamoto', 500),           // contained → 0.8
+      null, 0, 0,
+    )
+
+    // A should score higher due to confidence differential
+    expect(result.botAScore).toBeGreaterThan(result.botBScore)
+    expect(result.winnerId).toBe('a1')
+  })
 })
 
 describe('calculateElo', () => {

server/src/engine/scoring.ts

@@ -131,12 +131,15 @@ export function scoreRound(
       const confA = Math.min(correctA, 1)
       const confB = Math.min(correctB, 1)
 
+      // Confidence differential: exact match (1.0) vs fuzzy (0.8) gives up to +1.0 bonus
+      const confDiff = confA - confB
+
       if (aFaster) {
-        scoreA = FACTUAL_FASTER_BASE + (1 - speedRatio) * SPEED_ADVANTAGE_MULTIPLIER + confA
+        scoreA = FACTUAL_FASTER_BASE + (1 - speedRatio) * SPEED_ADVANTAGE_MULTIPLIER + confA + Math.max(0, confDiff)
-        scoreB = FACTUAL_SLOWER_BASE + speedRatio * SPEED_RATIO_MULTIPLIER + confB * CONFIDENCE_BONUS
+        scoreB = FACTUAL_SLOWER_BASE + speedRatio * SPEED_RATIO_MULTIPLIER + confB * CONFIDENCE_BONUS + Math.max(0, -confDiff)
       } else {
-        scoreA = FACTUAL_SLOWER_BASE + speedRatio * SPEED_RATIO_MULTIPLIER + confA * CONFIDENCE_BONUS
+        scoreA = FACTUAL_SLOWER_BASE + speedRatio * SPEED_RATIO_MULTIPLIER + confA * CONFIDENCE_BONUS + Math.max(0, confDiff)
-        scoreB = FACTUAL_FASTER_BASE + (1 - speedRatio) * SPEED_ADVANTAGE_MULTIPLIER + confB
+        scoreB = FACTUAL_FASTER_BASE + (1 - speedRatio) * SPEED_ADVANTAGE_MULTIPLIER + confB + Math.max(0, -confDiff)
       }
     } else if (correctA > 0 && correctB === 0) {
       scoreA = ONE_CORRECT_WINNER_BASE + correctA * CONFIDENCE_BONUS