Clarify Best Value route confidence

FuelBoard/StationDetailView.swift

@@ -2,6 +2,12 @@ import SwiftUI
 import MapKit
 import UIKit
 
+/// DEBUG-only explanation of the Best Value numbers for one station. It carries
+/// no precise user coordinates and is never persisted.
+struct FuelSavingsJourneyDiagnostics {
+    let rows: [(label: String, value: String)]
+}
+
 /// Detail sheet for a tapped station (when Settings → Tap station = More info).
 /// Shows a small map preview, the address, distance from the current location,
 /// and a Directions button that opens Apple Maps — the same destination a direct
@@ -14,6 +20,7 @@ struct StationDetailView: View {
     var fuelSavings: FuelSavingsAvailability? = nil
     var fuelSavingsAmount: FuelSavingsFillAmount = FuelSavingsSettings.defaultPreferredAmount
     var savingsBaselineStationName: String? = nil
+    var journeyDiagnostics: FuelSavingsJourneyDiagnostics? = nil
     /// Whether this station is starred for the fuel currently being viewed, and
     /// the toggle that flips it. Passed from the list that opened the sheet, so
     /// the star here and the star on the row are the same fuel-scoped
@@ -86,6 +93,18 @@ struct StationDetailView: View {
                     }
                 }
 
+#if DEBUG
+                if let journeyDiagnostics {
+                    Section("Best Value Diagnostics") {
+                        ForEach(journeyDiagnostics.rows, id: \.label) { row in
+                            LabeledContent(row.label, value: row.value)
+                                .font(.caption)
+                                .monospacedDigit()
+                        }
+                    }
+                }
+#endif
+
                 Section {
                     Button {
                         if let url = station.mapsDirectionsURL {