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/Intents/BookmarkOnscreen.swift

@@ -0,0 +1,19 @@
+import SwiftUI
+import AppIntents
+
+extension View {
+    /// Marks `bookmark` as the on-screen entity via an `NSUserActivity`, so
+    /// Siri / Apple Intelligence can resolve references like "summarize this"
+    /// or "open this one" while the bookmark is being viewed.
+    ///
+    /// iOS 26.0 ships no SwiftUI `.appEntityIdentifier` modifier, so this uses
+    /// the supported path: the standard `.userActivity` modifier plus
+    /// `NSUserActivity.appEntityIdentifier` (from `AppEntityAnnotatable`).
+    func bookmarkOnscreen(_ bookmark: Bookmark) -> some View {
+        userActivity("com.magicive.marks.viewBookmark", element: bookmark.id) { id, activity in
+            activity.title = bookmark.displayTitle
+            activity.appEntityIdentifier = EntityIdentifier(for: BookmarkEntity.self, identifier: id)
+            activity.isEligibleForSearch = true
+        }
+    }
+}