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.