Stop Approx. distances beside routed values; add Best Value alert mode

Distances (the "Approx. 1.5 mi" report):
- Refuse junk CoreLocation fixes (invalid accuracy, or a coarse >300 m fix
  within 3 km of the position already held; a coarse fix in another city is
  real travel and is still accepted).
- Resolve ONE display basis per render through FuelStore.distanceDisplayOrigin,
  used by rows, radius filter, sorting, savings figures and the Live Activity,
  so a stray fix can no longer re-base the list outside the route cache's 600 m
  ring while the routed values sit unused in the cache.
- Treat an incomplete route set as a state: retry it after 30 s rather than the
  full 10-minute throttle, and repair a stale incomplete flag once every
  in-radius station has a route (nothing left to route, flag stuck down).
- Widget extension resolves the same origin for fetch, radius, ordering and
  labels, and keeps its own fix in fuelboard.widgetLocation instead of
  overwriting the app's GPS fix.

Alerts:
- Best Value as a distinct alert mode on the existing geofence/notification
  pipeline, with a persisted threshold; reverts to cheapest when the feature is
  unconfigured and is not selectable while disabled.

Diagnostics (DEBUG, gated): FB_DIST_FOCUS now prints FB-DIST-ORIGIN (candidate,
accuracy, anchor, offset, resolved branch, served origin), FB_FAKE_FIX feeds a
synthetic fix through the real guard, FB_STALE_INCOMPLETE forces the stale-flag
state, and the route-fill status is surfaced in Settings behind debug mode.

Shared/FuelSavings.swift

@@ -253,6 +253,11 @@ struct FuelSavingsRankedStation: Equatable {
     let pricePencePerLitre: Double
 }
 
+struct BestValueAlertCandidate: Equatable {
+    let station: FuelStation
+    let result: FuelSavingsResult
+}
+
 enum FuelSavingsError: Error, Equatable {
     case invalidMPG
     case invalidAmount
@@ -417,6 +422,64 @@ enum FuelSavingsCalculator {
             }
         }
     }
+
+    static func bestValueAlertCandidate(
+        stations: [FuelStation],
+        fuel: FuelType,
+        settings: FuelSavingsSettings,
+        amount: FuelSavingsFillAmount,
+        minimumNetSavingGBP: Double,
+        journeyMiles: (FuelStation) -> FuelSavingsJourney?
+    ) -> BestValueAlertCandidate? {
+        guard settings.isConfigured, minimumNetSavingGBP.isFinite else { return nil }
+        let sellers = stations.filter { $0.prices[fuel] != nil }
+        guard !sellers.isEmpty else { return nil }
+        let baseline = FuelSavingsEstimator.baselineStation(sellers, fuel: fuel, journeyMiles: journeyMiles)
+        var best: BestValueAlertCandidate?
+        var bestDistance = Double.greatestFiniteMagnitude
+        var bestPrice = Double.infinity
+        var baselineJourneyCache: FuelSavingsJourney?
+        var baselineJourneyLoaded = false
+        for station in sellers {
+            let journey = journeyMiles(station)
+            let baselineJourney: FuelSavingsJourney? = {
+                guard let baseline else { return nil }
+                if baseline.id == station.id { return journey }
+                if !baselineJourneyLoaded {
+                    baselineJourneyCache = journeyMiles(baseline)
+                    baselineJourneyLoaded = true
+                }
+                return baselineJourneyCache
+            }()
+            guard case .available(let result) = FuelSavingsEstimator.availability(
+                station: station,
+                fuel: fuel,
+                baseline: baseline,
+                settings: settings,
+                amount: amount,
+                journey: journey,
+                baselineJourney: baselineJourney
+            ), result.status == .worthTheDrive,
+               result.netSavingGBP >= minimumNetSavingGBP else { continue }
+            let distance = journey?.miles ?? .greatestFiniteMagnitude
+            let price = station.prices[fuel] ?? .infinity
+            guard let current = best else {
+                best = BestValueAlertCandidate(station: station, result: result)
+                bestDistance = distance
+                bestPrice = price
+                continue
+            }
+            if result.netSavingGBP > current.result.netSavingGBP ||
+                (result.netSavingGBP == current.result.netSavingGBP && distance < bestDistance) ||
+                (result.netSavingGBP == current.result.netSavingGBP && distance == bestDistance && price < bestPrice) ||
+                (result.netSavingGBP == current.result.netSavingGBP && distance == bestDistance && price == bestPrice && station.id < current.station.id) {
+                best = BestValueAlertCandidate(station: station, result: result)
+                bestDistance = distance
+                bestPrice = price
+            }
+        }
+        return best
+    }
 }
 
 /// Turns stations into savings estimates.