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.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelHistoryTests.swift

@@ -164,6 +164,29 @@ final class FuelHistoryTests: XCTestCase {
         XCTAssertEqual(pruned.count, days.count)
     }
 
+    // MARK: Chart-key average
+
+    func testAveragePenceEmptyIsNil() {
+        XCTAssertNil(FuelHistoryStore.averagePence([]))
+    }
+
+    func testAveragePenceSinglePoint() {
+        let d = FuelHistoryStore.date(fromDay: "2026-08-15")!
+        XCTAssertEqual(FuelHistoryStore.averagePence([PricePoint(date: d, pence: 156.7)]), 156.7)
+    }
+
+    func testAveragePenceMultiplePoints() {
+        let d1 = FuelHistoryStore.date(fromDay: "2026-08-15")!
+        let d2 = FuelHistoryStore.date(fromDay: "2026-08-16")!
+        // (153.9 + 156.7) / 2 = 155.3 — exact in binary? 153.9+156.7=310.6, /2=155.3
+        let avg = FuelHistoryStore.averagePence([
+            PricePoint(date: d1, pence: 153.9),
+            PricePoint(date: d2, pence: 156.7),
+        ])
+        XCTAssertNotNil(avg)
+        XCTAssertEqual(avg!, 155.3, accuracy: 0.0001)
+    }
+
     // MARK: Mirror URLs
 
     func testHistoryFileURLKeepsBaseLastSegment() {