Use PathIdentifiable for navigation identity checks

Source/Turbo/Navigator/Helpers/PathIdentifiable.swift

@@ -0,0 +1,15 @@
+import UIKit
+
+/// A view controller may conform to `PathIdentifiable` to expose the URL it represents.
+///
+/// `Navigator` uses this URL — instead of view-controller type equality — when deciding
+/// whether a proposed visit refers to the page already on top of the stack or to the page
+/// directly beneath it. Without this conformance, two instances of the same custom view
+/// controller class but with different URLs are indistinguishable to the navigator and may
+/// be incorrectly popped or replaced.
+///
+/// `Visitable` inherits from this protocol, so existing visitable view controllers satisfy
+/// it automatically via `initialVisitableURL`.
+public protocol PathIdentifiable: AnyObject {
+    var initialVisitableURL: URL { get }
+}

Source/Turbo/Navigator/NavigationHierarchyController.swift

@@ -148,8 +148,8 @@ class NavigationHierarchyController {
     private func visitingSamePage(on navigationController: UINavigationController,
                                   with controller: UIViewController,
                                   via proposal: VisitProposal) -> Bool {
-        if let visitable = navigationController.topViewController as? Visitable {
-            return visitable.initialVisitableURL.isSameLocation(as: proposal.url, pathProperties: proposal.properties)
+        if let pathIdentifiable = navigationController.topViewController as? PathIdentifiable {
+            return pathIdentifiable.initialVisitableURL.isSameLocation(as: proposal.url, pathProperties: proposal.properties)
         } else if let topViewController = navigationController.topViewController {
             return topViewController.isMember(of: type(of: controller))
         }
@@ -162,8 +162,8 @@ class NavigationHierarchyController {
         guard navigationController.viewControllers.count >= 2 else { return false }
 
         let previousController = navigationController.viewControllers[navigationController.viewControllers.count - 2]
-        if let previousVisitable = previousController as? VisitableViewController {
-            return previousVisitable.initialVisitableURL.isSameLocation(as: proposal.url, pathProperties: proposal.properties)
+        if let pathIdentifiable = previousController as? PathIdentifiable {
+            return pathIdentifiable.initialVisitableURL.isSameLocation(as: proposal.url, pathProperties: proposal.properties)
         }
         return type(of: previousController) == type(of: controller)
     }

Source/Turbo/Visitable/Visitable.swift

@@ -10,11 +10,10 @@ public protocol VisitableDelegate: AnyObject {
     func visitableDidRequestRefresh(_ visitable: Visitable)
 }
 
-public protocol Visitable: AnyObject {
+public protocol Visitable: AnyObject, PathIdentifiable {
     var visitableViewController: UIViewController { get }
     var visitableDelegate: VisitableDelegate? { get set }
     var visitableView: VisitableView { get }
-    var initialVisitableURL: URL { get }
     var currentVisitableURL: URL { get }
 
     func visitableDidRender()