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.

FuelBoardTests/Tests/FuelBoardSharedTests/SortModeNamesTests.swift

@@ -21,9 +21,11 @@ final class SortModeNamesTests: XCTestCase {
         XCTAssertEqual(SortMode.allCases.map(\.rawValue), ["cheapest", "closest", "worthTheDrive"])
     }
 
-    func testTheFeatureItselfIsStillCalledWorthTheDrive() {
-        // Only the sort was renamed — the status wording the feature produces
-        // keeps its own name.
+    func testTheVerdictWordingIsUnchanged() {
+        // The feature and its sort are both called "Best Value" in user-facing
+        // copy now (Settings toggle + narrative, the sheet's section header).
+        // FuelSavingsStatus's own wording describes a VERDICT rather than the
+        // feature, and is not shown in the UI, so it keeps the drive phrasing.
         XCTAssertEqual(FuelSavingsStatus.worthTheDrive.displayName, "Worth the drive")
         XCTAssertEqual(FuelSavingsStatus.notWorthTheDrive.displayName, "Not worth the drive")
     }