Keep a section's stack when the sidebar selection changes

In the split-view sidebar layout (macOS, and now iPad and the iPhone
Duo), the detail NavigationStack that a selection change replaces
writes an empty path through its binding on the way out, so going to
another section and back always landed on the section's root. macOS
already behaved this way before the router.

Only the selected section's stack is on screen and can pop, so path
writes for any other section are ignored.

Yattee/Services/Navigation/NavigationRouter.swift

@@ -44,7 +44,15 @@ final class NavigationRouter {
         let key = item.id
         return Binding(
             get: { self.stacks[key] ?? [] },
-            set: { self.stacks[key] = $0 }
+            set: {
+                // Only the selected section's stack is on screen, so only it
+                // edits its path (Back, swipe back). A NavigationSplitView
+                // detail stack that is being replaced on a selection change
+                // writes an empty path on its way out, which would throw
+                // away what the section had pushed.
+                guard key == self.selection.id else { return }
+                self.stacks[key] = $0
+            }
         )
     }