feat: add session score display and refactor Jenkinsfile

Score tracks X wins, O wins, draws in-memory for current session.
Jenkinsfile rewritten as scripted pipeline with named execute functions
and per-branch strategy closures (mirrors BasePipeline pattern).

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

public/style.css

@@ -75,6 +75,37 @@ h1 {
   border-color: #e94560;
 }
 
+.scoreboard {
+  display: flex;
+  justify-content: center;
+  gap: 2rem;
+  margin-bottom: 1.5rem;
+}
+
+.score-item {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  background: #16213e;
+  border: 2px solid #0f3460;
+  border-radius: 8px;
+  padding: 0.5rem 1.5rem;
+  min-width: 70px;
+}
+
+.score-label {
+  font-size: 0.85rem;
+  color: #a8dadc;
+  text-transform: uppercase;
+  letter-spacing: 1px;
+}
+
+.score-value {
+  font-size: 2rem;
+  font-weight: bold;
+  color: #e94560;
+}
+
 .reset-btn {
   padding: 0.75rem 2rem;
   font-size: 1rem;