onboarding: replay on a fresh/reinstalled install, not every launch

The keychain-first completed flag persists across reinstalls (same bundle id),
so a new install was treated as already-completed and onboarding silently
skipped while permissions still prompted. Add install identity:
- Store an install id in BOTH the app-own container defaults (wiped on
  reinstall, persists across launches) and keychain (survives reinstall).
- isFreshInstall() is true on first-ever launch or when the local id is
  missing/different from keychain (i.e. reinstalled); it seeds a fresh common
  id so the next launch of the same install is not 'fresh' again.
- shouldShowOnboarding() = not completed || fresh install -> onboarding now
  replays once per fresh/reinstall, never on ordinary launches.
Works on free SideStore (no app-group), using the app's own defaults domain
as the same-install signal.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -671,3 +671,15 @@ final class RoadDistanceCacheTests: XCTestCase {
         XCTAssertEqual(km, s.distanceKM(to: 51.6, lng2: -0.1), accuracy: 0.0001)
     }
 }
+
+// MARK: - Install identity
+
+final class InstallIdentityTests: XCTestCase {
+    func testFreshInstallIsStableAfterFirstCall() {
+        // The first call seeds the common install id (local == keychain), so
+        // any subsequent call in the same process must report NOT-fresh. This
+        // holds regardless of persisted keychain/defaults state from prior runs.
+        _ = FuelStore.isFreshInstall()
+        XCTAssertFalse(FuelStore.isFreshInstall())
+    }
+}