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.xcodeproj/xcshareddata/xcschemes/Marks.xcscheme

@@ -67,6 +67,17 @@
          </BuildableReference>
       </MacroExpansion>
       <Testables>
+         <TestableReference
+            skipped = "NO"
+            parallelizable = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "F96181AD21F4A6755214E19B"
+               BuildableName = "MarksTests.xctest"
+               BlueprintName = "MarksTests"
+               ReferencedContainer = "container:Marks.xcodeproj">
+            </BuildableReference>
+         </TestableReference>
       </Testables>
       <CommandLineArguments>
       </CommandLineArguments>