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.

FuelBoard/WatchSyncManager.swift

@@ -11,6 +11,9 @@ extension Notification.Name {
     /// Posted when "Accurate distances" is switched, so the browse layer can
     /// re-render and start a fill from where the user actually is.
     static let fuelBoardAccurateDistancesChanged = Notification.Name("FuelBoardAccurateDistancesChanged")
+    /// Posted whenever the road-distance fill starts, finishes, or is gated.
+    /// Settings → Debug subscribes so the tester can see why Approx. remains.
+    static let fuelBoardRoadDistanceDebugUpdated = Notification.Name("FuelBoardRoadDistanceDebugUpdated")
 }
 
 @MainActor