Widget diagnostics: beacon from makeEntry (keychain + relay) + app diagnostics section

The small-widget skeleton survived four resolution fixes; instrument the
widget instead of guessing. Every makeEntry now writes its entry state
(intent type, location source, station count, fuel, sort, favourites flag,
first station) to a keychain beacon (app-readable) and fire-and-forgets a
GET to the relay's /api/v1/widget-diag so its access log records whether a
widget timeline actually runs in the extension and what it produced.
Settings → Debug gains a Widget Diagnostics section: the keychain beacon +
the installed-widget inventory from WidgetCenter. Relay gains the
/api/v1/widget-diag route (204, log-only).

FuelBoardWidgets/FuelPriceWidget.swift

@@ -127,6 +127,36 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
     }
 
     private func makeEntry(configuration: Configuration) async -> FuelPriceEntry {
+        let entry = await makeEntryCore(configuration: configuration)
+        Self.writeDiagBeacon(entry: entry)
+        return entry
+    }
+
+    /// Fire-and-forget diagnostics beacon: writes the entry state to keychain
+    /// (app-readable) and GETs the relay so its access log records that a
+    /// widget timeline actually ran in the extension and what it produced.
+    /// Deliberately outside the timeline result — can never affect rendering.
+    private static func writeDiagBeacon(entry: FuelPriceEntry) {
+        let first = entry.stations.first
+        let json = """
+        {"intent":"\(Configuration.self)","source":"\(entry.locationSource)",\
+        "n":\(entry.stations.count),"fuel":"\(entry.fuel.rawValue)",\
+        "sort":"\(entry.sort.rawValue)","fav":\(entry.isFavourites),\
+        "station":"\(first?.name ?? "")","price":\(first?.prices[entry.fuel] ?? -1)}
+        """
+        FuelStore.saveWidgetDiag(json)
+        guard let url = URLComponents(
+            url: RelayFuelProvider().baseURL.appendingPathComponent("api/v1/widget-diag"),
+            resolvingAgainstBaseURL: false
+        )?.url else { return }
+        var request = URLRequest(url: url)
+        request.timeoutInterval = 2
+        Task {
+            _ = try? await URLSession.shared.data(for: request)
+        }
+    }
+
+    private func makeEntryCore(configuration: Configuration) async -> FuelPriceEntry {
         // Per-widget config: fuel + sort + distance come from THIS widget instance.
         let fuel = FuelType(rawValue: configuration.fuel.rawValue) ?? .e10
         let isFavourites = configuration.sort == .favourites