Best Value: rename the remaining user-facing copy, hide the sheet section when off

Following the sort-segment rename, the rest of the user-facing "Worth the Drive"
copy moves to "Best Value".

- Settings: the toggle is now just "Best Value" (was "Enable Worth the Drive"),
  and the narrative footer opens "Best Value estimates fuel cost only, …".
- More info sheet: the section header is "Best Value" (was "Worth the drive?"),
  and the section itself is now conditional. `fuelSavings` is nil whenever the
  feature is off — `savingsLookup` returns nil when it is not configured — so
  the heading is never left dangling over an empty section while the feature is
  disabled.
- The unavailable text reads "Best Value unavailable: …".
- Comments and doc comments that named the sort were updated so they match the
  UI instead of contradicting it.

Deliberately unchanged: the enum's raw value stays `worthTheDrive`, so stored
sort preferences and widget configs keep decoding, and FuelSavingsStatus keeps
its verdict wording ("Worth the drive" / "Not worth the drive") — that describes
a station's verdict rather than the feature, and is not shown in the UI.

Verified on the simulator: with the feature OFF the more info sheet shows
Address / Distance / Directions and no heading at all; with it ON the sheet
shows the "Best Value" section; Settings reads "Fuel Savings" / "Best Value"
with no "Enable" anywhere. 172 tests, 0 failures.

FuelBoard/ContentView.swift

@@ -340,7 +340,7 @@ struct ContentView: View {
     private static var lastDriveLogLine: String?
 
     /// One summary line per ranking pass (FB-DRIVE) so a device console shows
-    /// whether the Worth the Drive list is actually carrying savings for the
+    /// whether the Best Value list is actually carrying savings for the
     /// whole pool — the regression was silent otherwise.
     private func logWorthTheDriveSummary(_ ordered: [FuelStation],
                                          baseline: FuelStation?,
@@ -1357,7 +1357,7 @@ struct StationRow: View {
             VStack(alignment: .trailing, spacing: 2) {
                 if isTopResult {
                     // The badge always marks the CHEAPEST PUMP PRICE (never
-                    // "best saving"), so in Worth the Drive order — where the
+                    // "best saving"), so in Best Value order — where the
                     // top row is the best net saving — it may sit lower down
                     // the list, and says so. It sits ABOVE the price it
                     // annotates rather than beside the name, so it reads as