feat(intents): onscreen entity annotations + App Intents tests

- bookmarkOnscreen(_:) publishes the viewed bookmark as the on-screen
  entity (NSUserActivity.appEntityIdentifier) so Siri can resolve "this"
  / "summarize this one"; applied to the BrowserView sheets in
  BookmarksView and SearchView
- Add MarksTests (XCTest) covering entity mapping, EntityIdentifier,
  IntentRouter, and the real perform() of SearchMarksIntent and
  OpenBookmarkIntent; wired into the Marks scheme test phase

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Marks/Views/BookmarksView.swift

@@ -178,6 +178,7 @@ struct BookmarksView: View {
                 BrowserView(url: url, title: bookmark.displayTitle, claude: viewModel.claude, podcastPlayer: viewModel.podcastPlayer) {
                     await viewModel.archive(bookmark)
                 }
+                .bookmarkOnscreen(bookmark)
             }
         }
         .onChange(of: browsingBookmark) { _, new in