fix(nav): a page settling keeps a reload's overlay entry; an old entry's sub-view goes to its own (review round 4)

- useUrlMirror: a change before the user has done anything since the load is the page settling (the
  rail stepping off a playlist deleted since, Files off a folder gone), not another view: the reload
  mark stays, and the player that reopens takes its entry over — one Back closes it, as before.
- useHistorySubview: an entry an older build wrote is a module's only when its _sub is the view the
  address names under that module's key (the card's push kept the season's `v` beside its `m`).
- Playlists: the rail's default selection and the page's "the rail moved there" read one helper.
- moduleUrl contract: writer literals in a choice's branches are scanned too; the sub-view census
  walks type arguments of any depth and refuses an aliased import.
- A stale comment on the nav press over a live search.

frontend/src/modules/playlists/Playlists.tsx

@@ -65,6 +65,7 @@ import NavGrid from "../../components/ui/NavGrid";
 import { PageToolbar } from "../../components/ui/PageShell";
 import { usePlaylistActions } from "./usePlaylistActions";
 import { usePlaylists } from "./playlistsContext";
+import { railDefaultId } from "./playlistsInit";
 import { usePref } from "../../lib/usePref";
 import { navItemAttrs } from "../../lib/listNav";
 import { ColumnHead } from "../../components/ui/tableHeader";
@@ -593,7 +594,7 @@ function PlaylistDetail() {
   const linkedOver = linkedId;
   const linkedOutOfReach =
     listQuery.data != null && !listQuery.data.some((p) => p.id === linkedOver);
-  const railMovedTo = listQuery.data?.[0]?.id ?? null;
+  const railMovedTo = railDefaultId(listQuery.data);
   const linkedListIn =
     linkedOutOfReach ||
     (detail != null && (detail.items.length === 0 || items.length > 0)) ||