fix(nav): the address keeps what the app rewrites around it (review round 3)

- The Feed press over a live search ends only the search: the feed's own view stays named.
- Leaving a search steps back only off an entry the app pushed (_ytOwn); one a link or a reload
  opened is cleared in place, also where the Navigation API is missing.
- useUrlMirror rewrites a changed view through replaceView, so a reload's overlay marks go; its
  first write and its landing writes keep them for an overlay that reopens.
- Files and Playlists: a box emptied and filled again settles at once, never naming the ended search.
- Playlists: a linked player is scoped to the playlist the store started with, and only the rail's
  own move to the first playlist is no pick.
- Contract: writer objects are scanned to their matching brace, a computed or shorthand key is
  refused, and every useHistorySubview call hands a named definition.
- Lint: playwright/no-skipped-test also refuses fixme (gate snapshot regenerated).
- FEED_DEFAULT_SORT at the three remaining sites; linkedNotListed says to search by name.

frontend/src/features/navigation/NavigationProvider.ytSearch.dom.test.tsx

@@ -243,6 +243,33 @@ describe("NavigationProvider · the YouTube search in the address", () => {
     expect(window.history.length).toBe(depth + 1); // its own entry: Back returns to the search
   });
 
+  it("the Feed press on the search keeps the feed's own view in the address", () => {
+    window.history.replaceState({ sfPage: "feed" }, "", "/?page=feed&show=all&sort=oldest");
+    const { result } = render();
+    act(() => result.current.enterYtSearch("lofi"));
+    act(() => result.current.setPage("feed"));
+    expect(search().get("yt")).toBeNull();
+    expect([search().get("show"), search().get("sort")]).toEqual(["all", "oldest"]);
+  });
+
+  it("a search sent from the box is left by a step back off its own entry", () => {
+    const { result } = render();
+    act(() => result.current.enterYtSearch("lofi"));
+    const back = vi.spyOn(window.history, "back").mockImplementation(() => {});
+    act(() => result.current.exitYtSearch());
+    expect(back).toHaveBeenCalledTimes(1);
+  });
+
+  it("a search a link opened is left in place, not by a step back off the document", () => {
+    window.history.replaceState(null, "", "/?page=feed&yt=lofi");
+    const { result } = render();
+    const back = vi.spyOn(window.history, "back");
+    act(() => result.current.exitYtSearch());
+    expect(back).not.toHaveBeenCalled();
+    expect(result.current.ytSearch).toBeNull();
+    expect(search().get("yt")).toBeNull();
+  });
+
   it("a landing on a channel opened from the search (an overlay over it closing) keeps the search", () => {
     const { result } = render();
     act(() => result.current.enterYtSearch("lofi"));