Make station price age informational only

The relay's price_updated is the maximum price_last_updated across the grades it maps, so it cannot vouch for the selected grade: stop using it to score, gate or warn about Best Value.

- FuelPriceFreshness is now station-level and display-only: one age per station, fresh/outdated/unknown/invalid, "May be outdated" past 30 days, with future-tolerance and non-finite guards.
- Best Value restores the closest-station-selling-the-selected-fuel baseline and its savings ranking; eligibility gating, eligible-first sorting and the unavailable banner are gone.
- Price age appears in the More info sheet only - no listing-row warnings, no extra banners.
- Copy reverted from "closest eligible station" to "closest station"; stale price-confidence strings removed, "May be outdated" added.
- Dormant per-grade timestamp plumbing removed, and regression tests added for every fuel type, missing/invalid/future stamps and the bundled offline dump.

FuelBoard/HowBestValueSheet.swift

@@ -14,9 +14,9 @@ struct HowBestValueSheet: View {
             List {
                 Section {
                     VStack(alignment: .leading, spacing: 10) {
-                        Text("Best Value compares each station with the closest eligible station by journey distance. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.")
+                        Text("Best Value compares each station with the closest station by journey distance. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.")
                         Text("Your fill-up amount and MPG are used to show whether you could save overall.")
-                        Text("With a £ budget you spend that amount at each station; the litres differ by station. Savings compare those litres with the same quantity at your closest eligible station, then deduct extra journey fuel. You do not receive cash back.")
+                        Text("With a £ budget you spend that amount at each station; the litres differ by station. Savings compare those litres with the same quantity at your closest station, then deduct extra journey fuel. You do not receive cash back.")
                         Text("With Litres, every station is compared for the fixed quantity you selected.")
                         Text("Results are marked “Approx.” until road distances are available.")
                             .foregroundStyle(.secondary)