Store: save one Fuel Savings field per edit, and update keychain items in place

The Settings sliders called persistFuelSavingsSettings() on every value change,
which rewrote all four keys — four UserDefaults writes and four keychain round
trips per step, on the main thread, up to 90 times per drag. That is what made
them stutter.

saveFuelSavingsField(_:from:) writes just the field that moved, so a slider
gesture touches one key.

writeKeychain now uses SecItemUpdate and only falls back to SecItemAdd on
errSecItemNotFound. Delete-then-add cost two round trips to securityd and
briefly left the key holding no value at all — a window where the keychain, the
authoritative mirror on read, could disagree with the app-group defaults.

Tests pin the store half: a single-field save leaves the other three keys alone,
and the whole-settings save still writes all four.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -357,6 +357,68 @@ final class AlertsFuelTests: XCTestCase {
     }
 }
 
+/// The Settings sliders save ONE field per gesture instead of rewriting all four
+/// keys (four UserDefaults writes plus four keychain round trips per drag is what
+/// made them stutter). These pin the store half of that: a single-field save must
+/// leave the other three keys exactly as they were.
+final class FuelSavingsFieldSaveTests: XCTestCase {
+    private func seed() {
+        FuelStore.saveFuelSavingsSettings(FuelSavingsSettings(
+            enabled: true,
+            mpg: 40,
+            preferredAmount: FuelSavingsFillAmount(value: 50, unit: .pounds)
+        ))
+    }
+
+    func testSavingMPGLeavesTheOtherFieldsAlone() {
+        seed()
+        var edited = FuelStore.loadFuelSavingsSettings()
+        edited.mpg = 62
+        FuelStore.saveFuelSavingsField(.mpg, from: edited)
+        let reloaded = FuelStore.loadFuelSavingsSettings()
+        XCTAssertEqual(reloaded.mpg, 62, "the edited field is written")
+        XCTAssertTrue(reloaded.enabled, "enabled survives an MPG save")
+        XCTAssertEqual(reloaded.preferredAmount.value, 50, "the fill-up amount survives an MPG save")
+        XCTAssertEqual(reloaded.preferredAmount.unit, .pounds, "the unit survives an MPG save")
+    }
+
+    func testSavingFillAmountLeavesMPGAndUnitAlone() {
+        seed()
+        var edited = FuelStore.loadFuelSavingsSettings()
+        edited.preferredAmount.value = 75
+        FuelStore.saveFuelSavingsField(.amount, from: edited)
+        let reloaded = FuelStore.loadFuelSavingsSettings()
+        XCTAssertEqual(reloaded.preferredAmount.value, 75, "the edited field is written")
+        XCTAssertEqual(reloaded.mpg, 40, "MPG survives a fill-up save")
+        XCTAssertEqual(reloaded.preferredAmount.unit, .pounds, "the unit survives a fill-up save")
+    }
+
+    func testSavingEnabledLeavesTheNumbersAlone() {
+        seed()
+        var edited = FuelStore.loadFuelSavingsSettings()
+        edited.enabled = false
+        FuelStore.saveFuelSavingsField(.enabled, from: edited)
+        let reloaded = FuelStore.loadFuelSavingsSettings()
+        XCTAssertFalse(reloaded.enabled, "the edited field is written")
+        XCTAssertEqual(reloaded.mpg, 40, "MPG survives an enabled save")
+        XCTAssertEqual(reloaded.preferredAmount.value, 50, "the fill-up amount survives an enabled save")
+    }
+
+    func testWholeSettingsSaveStillWritesEveryField() {
+        seed()
+        FuelStore.saveFuelSavingsSettings(FuelSavingsSettings(
+            enabled: false,
+            mpg: 55,
+            preferredAmount: FuelSavingsFillAmount(value: 30, unit: .litres)
+        ))
+        let reloaded = FuelStore.loadFuelSavingsSettings()
+        XCTAssertFalse(reloaded.enabled)
+        XCTAssertEqual(reloaded.mpg, 55)
+        XCTAssertEqual(reloaded.preferredAmount.value, 30)
+        XCTAssertEqual(reloaded.preferredAmount.unit, .litres)
+    }
+}
+
 final class AlertRadiusFollowTests: XCTestCase {
     func testAlertRadiusOptionsAreMileFriendly() {
         // 1–2 city, 3 town, 5 default, 8 motorway — the geofence ceiling.

Shared/FuelStore.swift

@@ -850,6 +850,26 @@ struct FuelStore {
         saveString(settings.preferredAmount.unit.rawValue, service: fuelSavingsUnitKey)
     }
 
+    /// One field of the Fuel Savings settings. A control that edits a single
+    /// value saves just that field, so moving the MPG slider rewrites the MPG key
+    /// instead of all four — four UserDefaults writes and four keychain round
+    /// trips per gesture is what made those sliders stutter.
+    enum FuelSavingsField {
+        case enabled
+        case mpg
+        case amount
+        case unit
+    }
+
+    static func saveFuelSavingsField(_ field: FuelSavingsField, from settings: FuelSavingsSettings) {
+        switch field {
+        case .enabled: saveString(settings.enabled ? "1" : "0", service: fuelSavingsEnabledKey)
+        case .mpg: saveString(String(settings.mpg), service: fuelSavingsMPGKey)
+        case .amount: saveString(String(settings.preferredAmount.value), service: fuelSavingsAmountKey)
+        case .unit: saveString(settings.preferredAmount.unit.rawValue, service: fuelSavingsUnitKey)
+        }
+    }
+
     /// Superscript digit glyphs for the pounds & pence format's small raised
     /// third digit (the forecourt style: "£1.29⁹").
     private static let superscriptDigits: [Character] = ["⁰", "¹", "²", "³", "⁴", "⁵", "⁶", "⁷", "⁸", "⁹"]
@@ -1719,11 +1739,18 @@ struct FuelStore {
     }
 
     private static func writeKeychain(data: Data, service: String) {
-        let deleteQuery: [String: Any] = [
+        // Update in place when the item already exists. Delete-then-add cost two
+        // round trips to securityd and briefly left the key holding no value at
+        // all, which is half of why a setting could disagree between launches.
+        let query: [String: Any] = [
             kSecClass as String: kSecClassGenericPassword,
             kSecAttrService as String: service,
         ]
-        SecItemDelete(deleteQuery as CFDictionary)
+        let status = SecItemUpdate(query as CFDictionary,
+                                   [kSecValueData as String: data] as CFDictionary)
+        guard status == errSecItemNotFound else { return }
+        // Nothing stored under this service yet — the first write after the app
+        // was deleted, or a key added since. Add it.
         let addQuery: [String: Any] = [
             kSecClass as String: kSecClassGenericPassword,
             kSecAttrService as String: service,