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.

src/routes/players.js

@@ -495,7 +495,8 @@ router.post('/api/calculate-target-rating/:pdgaNumber', async (req, res) => {
       requiredAverage: result.requiredAverage,
       predictedRating: result.simulatedPredicted,
       warning: result.warning,
-      iterations: result.iterations
+      iterations: result.iterations,
+      sensitivity: result.sensitivity
     });
   } catch (err) {
     logger.error(`Target rating calc failed for PDGA ${pdgaNum}: ${err.message}`);

src/services/target-rating-calculator.js

@@ -58,6 +58,14 @@ function calculateRequiredAverage(roundRatings, targetRating, numRounds) {
     warning = 'Could not converge precisely; result is approximate.';
   }
 
+  const lowerAvg = Math.max(400, Math.round((requiredAverage - 1) * 10) / 10);
+  const upperAvg = Math.min(1200, Math.round((requiredAverage + 1) * 10) / 10);
+  const sensitivity = {
+    lower: { average: lowerAvg, predicted: simulate(lowerAvg) },
+    target: { average: requiredAverage, predicted: simulatedPredicted },
+    upper: { average: upperAvg, predicted: simulate(upperAvg) }
+  };
+
   logger.debug(`Target rating calc: target=${targetRating} rounds=${numRounds} → avg=${requiredAverage} (iterations=${iterations}, simulated=${simulatedPredicted})`);
 
   return {
@@ -65,7 +73,8 @@ function calculateRequiredAverage(roundRatings, targetRating, numRounds) {
     currentPredicted,
     simulatedPredicted,
     iterations,
-    warning
+    warning,
+    sensitivity
   };
 }