feat: show sensitivity bracket around required average

After the binary search converges, also simulate predicted rating at
required±1 average. Display the three rows in the modal so the user can
see how sharp the requirement is — e.g. whether averaging 1 point lower
costs them 1 point of predicted rating or 5.

public/css/players.css

@@ -360,3 +360,24 @@
     gap: 12px;
     align-items: flex-start;
 }
+
+.target-rating-result .sensitivity {
+    margin-top: 12px;
+    padding-top: 12px;
+    border-top: 1px dashed var(--border);
+}
+
+.target-rating-result .sensitivity-heading {
+    font-size: 0.9em;
+    color: var(--text-muted);
+    margin-bottom: 4px;
+}
+
+.target-rating-result .sensitivity-row {
+    font-variant-numeric: tabular-nums;
+    padding-left: 12px;
+}
+
+.target-rating-result .sensitivity-row.is-target {
+    font-weight: 600;
+}

public/js/players.js

@@ -647,6 +647,10 @@ async function calculateTargetRating(event) {
         mutedLine.textContent = 'Across ' + data.rounds + ' synthetic rounds before the next PDGA update.';
         summary.appendChild(mutedLine);
 
+        if (data.sensitivity) {
+            renderSensitivity(data.sensitivity, summary);
+        }
+
         if (data.warning) {
             _targetResultMsg(summary, 'warning', data.warning);
         }
@@ -666,6 +670,30 @@ function closeTargetRatingModal(event) {
     document.getElementById('target-rating-modal').style.display = 'none';
 }
 
+function renderSensitivity(sensitivity, container) {
+    const wrapper = document.createElement('div');
+    wrapper.className = 'sensitivity';
+
+    const heading = document.createElement('div');
+    heading.className = 'sensitivity-heading';
+    heading.textContent = 'Sensitivity:';
+    wrapper.appendChild(heading);
+
+    const rows = [
+        { row: sensitivity.lower, isTarget: false },
+        { row: sensitivity.target, isTarget: true },
+        { row: sensitivity.upper, isTarget: false }
+    ];
+    for (const { row, isTarget } of rows) {
+        const line = document.createElement('div');
+        line.className = 'sensitivity-row' + (isTarget ? ' is-target' : '');
+        line.textContent = 'Average ' + row.average + ' → predicted ' + row.predicted + (isTarget ? ' (target)' : '');
+        wrapper.appendChild(line);
+    }
+
+    container.appendChild(wrapper);
+}
+
 function renderNoHistoryPrompt(pdgaNumber, container) {
     const wrapper = document.createElement('div');
     wrapper.className = 'no-history-prompt';