fix: lower critical hit threshold from 4 to 3

Both-correct fights could almost never produce critical hits because
max margin at typical speed differences was ~2.2, far below threshold
of 4. Lowering to 3 yields ~15% critical hit rate (target 10-20%),
making speed differences produce more exciting fight dynamics.

Research findings: simulated 1000 fights with both bots answering
correctly. Critical rate went from 0% to 15.3% with new threshold.

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

server/src/engine/scoring-analysis.test.ts

@@ -207,7 +207,7 @@ describe('scoring competitiveness analysis', () => {
     }
 
     section.push('')
-    section.push('**Critical hit threshold: 4**')
+    section.push('**Critical hit threshold: 3**')
     section.push('')
     reportLines.push(...section)