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.

FuelBoard/TrendsView.swift

@@ -175,6 +175,7 @@ struct TrendsView: View {
                 VStack(spacing: 12) {
                     chart
                     legend
+                    legendFooter
                 }
             }
         }
@@ -256,10 +257,41 @@ struct TrendsView: View {
                     Text(history.name)
                         .font(.caption)
                         .lineLimit(1)
+                    if let avg = FuelHistoryStore.averagePence(history.points) {
+                        Text(legendFigure(avg))
+                            .font(.caption)
+                            .foregroundStyle(.secondary)
+                            .monospacedDigit()
+                    }
                     Spacer()
                 }
             }
         }
         .padding(.horizontal, 4)
     }
+
+    /// The bracket figure in the chart key: absolute pence in Price mode,
+    /// signed pence above the day's cheapest in vs-cheapest mode — always
+    /// pence, matching the list rows (the y-axis follows the display toggle).
+    private func legendFigure(_ pence: Double) -> String {
+        switch mode {
+        case .price:
+            return String(format: "%.1fp", pence)
+        case .vsCheapest:
+            return pence > 0 ? String(format: "+%.1fp", pence) : String(format: "%.1fp", pence)
+        }
+    }
+
+    /// One-line descriptor under the key so the brackets are self-explanatory.
+    private var legendFooter: some View {
+        Group {
+            if mode == .price {
+                Text("Average price over the days shown")
+            } else {
+                Text("Average pence above the day's cheapest favourite")
+            }
+        }
+        .font(.caption2)
+        .foregroundStyle(.secondary)
+    }
 }