Trends: chart key shows average price per station + descriptor footer

Legend rows now show each station's average over the days shown
in brackets (pence; signed +X.Xp in vs-cheapest mode, matching the
list-row deltas). Footer under the key explains the figure per mode.
averagePence helper (nil for empty series) + 3 tests; -showTrends
becomes a permanent QA hook like -showKeySheet.

Shared/FuelHistory.swift

@@ -148,6 +148,13 @@ enum FuelHistoryStore {
         }
     }
 
+    /// Average pence over a series' points; nil when there are no points
+    /// (a station with no data shows no bracket figure in the chart key).
+    static func averagePence(_ points: [PricePoint]) -> Double? {
+        guard !points.isEmpty else { return nil }
+        return points.reduce(0) { $0 + $1.pence } / Double(points.count)
+    }
+
     /// Rebase every station's series so each day's CHEAPEST favourite sits at
     /// 0 and the others show signed pence above it (mirrors the list's
     /// baseline delta pattern). Days where a station has no point are gaps.