How Best Value works: its own sheet, opened from the Key sheet and More info sheet

The explanation used to be a section inside the Key sheet. It is a standalone
HowBestValueSheet now, opened from a tappable row with a chevron: at the bottom of
the Key sheet, and at the bottom of the More info sheet below the Best Value card.

Gating: the More info row sits INSIDE the same `if let fuelSavings` as the card it
follows, so it disappears with the feature. Verified with Best Value toggled off in
Settings: the More info sheet then shows neither the card nor the row. The Key
sheet row is deliberately ungated — that sheet is the legend and is reachable
whatever the feature is doing.

Copy is unchanged: the same Localizable.strings strings are reused, and the old
section header became the sheet's title. A DEBUG-only `-showBestValueInfo` hook
opens the sheet without a tap.

FuelBoard/StationsView.swift

@@ -35,6 +35,11 @@ struct StationsView: View {
     /// info button in the header — keeps the list focused on stations.
     @State private var showKey = false
 
+    /// The "How Best Value works" sheet. The explanation used to be a section in
+    /// the Key sheet; it is its own sheet now, opened from a row at the bottom of
+    /// the Key sheet and from the More info sheet.
+    @State private var showBestValueInfo = false
+
     /// Station picked for the More-info sheet (tap action = showDetails).
     @State private var detailStation: FuelStation?
 
@@ -218,6 +223,18 @@ struct StationsView: View {
                 if ProcessInfo.processInfo.arguments.contains("-showKeySheet") {
                     showKey = true
                 }
+                // QA hook: `-showBestValueInfo` opens the Key sheet and then the
+                // explanation on top of it, so the copy can be screenshotted
+                // without a tap. It has to chain: the explanation's `.sheet` lives
+                // inside the Key sheet's content, so it cannot present while the
+                // Key sheet is shut.
+                if BestValueInfoQA.isRequested {
+                    showKey = true
+                    Task { @MainActor in
+                        try? await Task.sleep(nanoseconds: 1_200_000_000)
+                        showBestValueInfo = true
+                    }
+                }
 #endif
             }
             .task(id: stations.count) {
@@ -299,15 +316,12 @@ struct StationsView: View {
                             .listRowInsets(EdgeInsets(top: 4, leading: 16, bottom: 4, trailing: 16))
                         }
 
-                        Section("How Best Value works") {
-                            VStack(alignment: .leading, spacing: 10) {
-                                Text("Best Value compares each station with the closest one. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.")
-                                Text("Your fill-up amount and MPG are used to show whether you could save overall.")
-                                Text("Results are marked “Approx.” until road distances are available.")
-                                    .foregroundStyle(.secondary)
-                            }
-                            .font(.subheadline)
-                            .padding(.vertical, 4)
+                        // The explanation is its own sheet now, opened from this
+                        // row. Deliberately NOT gated by Best Value: the Key sheet
+                        // is the legend and is reachable whatever the feature is
+                        // doing.
+                        Section {
+                            HowBestValueRow { showBestValueInfo = true }
                         }
                     }
                     .navigationTitle("Key")
@@ -317,6 +331,8 @@ struct StationsView: View {
                             Button("Done") { showKey = false }
                         }
                     }
+                    // Nested sheet: the Key sheet stays behind the explanation.
+                    .sheet(isPresented: $showBestValueInfo) { HowBestValueSheet() }
                 }
                 .presentationDetents([.fraction(0.6)])
                 .presentationBackground(Color(UIColor.systemGroupedBackground))