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/StationDetailView.swift

@@ -33,6 +33,9 @@ struct StationDetailView: View {
     /// the caption fades out on its own.
     @State private var showingCopied = false
     @State private var copyResetTask: Task<Void, Never>?
+    /// The "How Best Value works" sheet, opened from the row at the bottom of the
+    /// Best Value card (the Key sheet opens the same shared view).
+    @State private var showBestValueInfo = false
 
     private let reportIssueURL = URL(string: "https://www.gov.uk/guidance/report-an-error-in-fuel-prices-or-forecourt-details")!
 
@@ -240,6 +243,13 @@ struct StationDetailView: View {
                     } header: {
                         Text("Best Value")
                     }
+
+                    // The explanation lives in its own sheet now, opened from this
+                    // row. The row sits INSIDE the `if let` so it carries the same
+                    // gate as the card above: no card, no row.
+                    Section {
+                        HowBestValueRow { showBestValueInfo = true }
+                    }
                 }
 
 #if DEBUG
@@ -265,6 +275,8 @@ struct StationDetailView: View {
             } message: {
                 Text("Are you sure you want to report an error in fuel prices or forecourt details? This will open in your chosen browser.")
             }
+            // Nested sheet: the More info sheet stays put behind the explanation.
+            .sheet(isPresented: $showBestValueInfo) { HowBestValueSheet() }
             .toolbar {
                 ToolbarItem(placement: .cancellationAction) {
                     Button("Done") { dismiss() }