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).

Shared/FuelStore.swift

@@ -696,4 +696,16 @@ struct FuelStore {
         UserDefaults(suiteName: appGroupSuite)?.set(value, forKey: service)
         writeKeychain(data: Data(value.utf8), service: service)
     }
+
+    // Widget diagnostics beacon — the widget extension writes its last
+    // makeEntry state here (keychain survives on free SideStore accounts where
+    // the app-group container isn't provisioned); the app's Debug section
+    // reads it to see whether a widget timeline ran and what it produced.
+    static func saveWidgetDiag(_ json: String) {
+        saveString(json, service: "widget.diag")
+    }
+
+    static func loadWidgetDiag() -> String? {
+        loadString(service: "widget.diag")
+    }
 }