P1: Favourites Trends graph — FuelHistoryStore + TrendsView (Swift Charts)

- Shared/FuelHistory.swift: GitHub mirror read path (latest.json pointer +
  history/YYYY-MM-DD.json day files); day math (UTC noon keys), slim day decode
  with the shared price band, series + deltaSeries (vs-cheapest rebase),
  favourites-only app-group cache (90-day prune), parallel per-day fetches,
  404/network = graph gap never error.
- FuelPriceProvider: shared priceBand + mapGrades (single source for live +
  history decoding).
- FuelBoard/TrendsView.swift: line chart in the Favourites tab via toolbar
  button + sheet; fuel capsule (fuels with favourites only), 7/30/90 range,
  Price/vs-cheapest toggle, per-station legend, price-display style on y-axis,
  empty/loading/retry states with honest copy; no widget in v1.
- Localizable.strings: Trends keys.
- 83 tests (13 history + URL regression): day math, band guard, series gaps,
  delta rebasing, prune, and the appendingPathComponent regression (relative
  URL resolution dropped /main — all fetches 404'd).
- Sim-verified: favourites rows + TOP/deltas; sheet controls + building-up
  state with live first-snapshot date (archive has 1 day; lines render once
  >=2 snapshots).
- Backlog: P1 Trends DONE (unmerged); P0 REMAINING = live provider chain +
  telemetry beacon.

Shared/FuelPriceProvider.swift

@@ -42,6 +42,28 @@ enum FuelPriceProvider {
         }
     }
 
+    /// Price sanity band (pence/litre) — anything outside is relay regression
+    /// garbage and must never reach calculations (shared by the live decode
+    /// and the history mirror decoder).
+    static let priceBand: ClosedRange<Double> = 50...500
+
+    /// Maps relay grade keys (E5/E10/DIESEL…) to FuelType with the defensive
+    /// band guard. Shared by the relay decode and the history mirror parser so
+    /// both surfaces apply identical sanitisation.
+    static func mapGrades(_ prices: [String: Double]) -> [FuelType: Double] {
+        var result: [FuelType: Double] = [:]
+        for (grade, value) in prices {
+            guard priceBand.contains(value) else { continue }
+            switch grade.uppercased() {
+            case "E10": result[.e10] = value
+            case "E5": result[.e5] = value
+            case "DIESEL", "B7", "B7S", "B7P", "B10": result[.diesel] = value
+            default: break
+            }
+        }
+        return result
+    }
+
     /// Decodes the relay envelope metadata (source, dataset update time) — the
     /// About section shows these so the user can see which data source is
     /// live and how fresh the GOV.UK data itself is. The fields are additive
@@ -143,20 +165,9 @@ private struct RelayResponse: Codable {
         /// regression (e.g. the band being removed server-side) can't re-poison
         /// the nationwide cheapest reference with 1.3p / 1589p garbage.
         var allPrices: [FuelType: Double] {
-            var result: [FuelType: Double] = [:]
-            if let prices {
-                for (grade, value) in prices {
-                    guard (50...500).contains(value) else { continue }
-                    switch grade.uppercased() {
-                    case "E10": result[.e10] = value
-                    case "E5": result[.e5] = value
-                    case "DIESEL", "B7", "B7S", "B7P", "B10": result[.diesel] = value
-                    default: break
-                    }
-                }
-            }
+            var result = FuelPriceProvider.mapGrades(prices ?? [:])
             // Backwards-compat: relay versions without `prices` still send `price`.
-            if result.isEmpty, let price, (50...500).contains(price) {
+            if result.isEmpty, let price, FuelPriceProvider.priceBand.contains(price) {
                 result[.e10] = price
             }
             return result