Worth the Drive: calculation audit + listing/detail polish

Calculation audit (brief config: 30 MPG imperial, £60 fill-up, baseline
Rontec Shelf 171.9p at 0.4 mi):

- One mile<->kilometre constant pair (kilometresPerMile 1.609344,
  metresPerMile 1609.344) replaces the mixed 1609.34 / 0.621371 / 1.60934
  used in the savings path. Effect ~2 ppm (invisible), but the conversions
  are now auditable in one place.
- New FuelSavingsCalculationAuditTests (20 deterministic tests, no live
  MKDirections) pins the brief's three stations - Tesco 170.9p/2.0 mi,
  MFG Hare & Hounds 172.9p/2.8 mi, PGG Napoleon 171.9p/2.8 mi - plus
  imperial-vs-US gallon, MPG and budget scaling, the break-even tolerance
  boundary, the nearer-candidate clamp, route-unavailable, mixed-basis
  estimates and display-rounding boundaries.
- Formula unchanged, and verified correct: fixed quantity (budget / baseline
  price), pump saving, round-trip extra distance clamped at zero, travel fuel
  priced at the candidate station, +/-5p break even, baseline labelled
  "Closest station", sorting on the unrounded net saving. The brief's "£0.18"
  for Tesco is the rounded 1 dp extra-distance caption (2.02-2.05 mi reads as
  "2.0 mi"), not a rounding bug in the money.

UI (uncommitted since d7a0ecb):

- Done bar drawn in the bottom safe area from keyboard notifications:
  .toolbar(placement: .keyboard) is device-dependent (present on the 15 Pro,
  absent on the SE on the same iOS build).
- Favourite star in the More-info heading, wired to the same fuel-scoped
  toggle as the listing rows at all three call sites.
- CHEAPEST/TOP badge moved above the price so it reads as part of it.
- Listing rows carry the real session fill-up amount instead of a hard zero
  (the compact banner does not print it today; a future row layout would).

153 tests, 0 failures. Simulator and device builds succeed.

Shared/FuelSavings.swift

@@ -100,6 +100,25 @@ enum FuelSavingsFormat {
     static func money(_ value: Double) -> String {
         String(format: "£%.2f", value)
     }
+
+    /// A distance stored in MILES, formatted in the display unit. The figures
+    /// behind an estimate are stored in miles whatever the user's unit, so this
+    /// is the one place that conversion lives.
+    static func distance(_ miles: Double, unit: DistanceUnit) -> String {
+        // Convert miles→km FIRST, then let the unit format it. `unit.toKM` would
+        // convert by the CURRENT unit instead, leaving a km user's figure
+        // unlabelled and 1.6× too large.
+        unit.format(miles * FuelSavingsCalculator.kilometresPerMile)
+    }
+
+    /// Row label for the extra driving a saving has to beat. The stored figure is
+    /// always MILES while the row's distance line is one-way, so the wording says
+    /// "more driving": the extra is an out-and-back figure and can legitimately
+    /// exceed the one-way distance, which read as a contradiction before.
+    static func extraTravelLabel(extraDistanceMiles: Double, unit: DistanceUnit) -> String? {
+        guard extraDistanceMiles > 0.05 else { return nil }
+        return "\(distance(extraDistanceMiles, unit: unit)) more driving than closest"
+    }
 }
 
 extension FuelSavingsResult {
@@ -140,6 +159,15 @@ extension FuelSavingsResult {
             return "\(FuelSavingsFormat.money(abs(netSavingGBP))) worse off after travel\(estimate)"
         }
     }
+
+    /// Row capsule copy: the headline, plus what the baseline row IS. Without the
+    /// suffix the baseline capsule just repeated the status word above it
+    /// ("Closest station" twice) and said nothing about the saving being measured
+    /// against it.
+    func capsuleHeadline() -> String {
+        let headline = headline(compact: true)
+        return status == .baseline ? "\(headline) · your baseline" : headline
+    }
 }
 
 /// One station's journey distance, plus how it was obtained.
@@ -173,7 +201,18 @@ enum FuelSavingsError: Error, Equatable {
 }
 
 enum FuelSavingsCalculator {
+    /// 1 UK (imperial) gallon. Must never be the US gallon (3.785411784 L):
+    /// UK pumps are priced per litre and drivers quote UK MPG, so a US gallon
+    /// would understate every travel cost by 17%.
     static let imperialGallonLitres = 4.54609
+    /// Exact mile↔kilometre factors (1 mile = 1.609344 km). Every figure the
+    /// savings maths keeps is in MILES, and every displayed or fallback
+    /// distance converts through these, so one quantity is never converted with
+    /// two different constants. The audit found 1.60934, 1609.34 and 0.621371
+    /// mixed in the same path — a ~2 parts-per-million difference, immaterial to
+    /// the money, but three constants where there should be one.
+    static let kilometresPerMile = 1.609344
+    static let metresPerMile = 1609.344
     static let breakEvenToleranceGBP = 0.05
     static let minimumMPG: Double = 5
     static let maximumMPG: Double = 200