app: implement Link/Unlink for timeline clips

编辑 > 链接/重新链接 (Cmd+L) was an unhandled stub
(PanelCommandHandler::toggle_links returned false), so linking clips
never created graph links and linked drags never happened.

- graphops::set_clips_linked: one undoable entry toggling every pair of
  the selection; the undo restores the exact prior internal topology.
- RealEngine::toggle_clip_links: fully-linked selections unlink,
  otherwise the selection links together (the C++ crude "any member has
  ANY link" rule would have made split halves — which inherit the
  original clip's A/V links — impossible to link to each other).
- TimelinePanel overrides toggle_links, routing the timeline selection.
- MockEngine stores demo link pairs.
- Test: link_unlink_toggles_the_selection_links — dropped A/V pair
  toggles off/on with undo, and split halves link manually.

crates/oak-app/src/panels/timeline.rs

@@ -683,6 +683,14 @@ impl<E: AppEngine> PanelCommandHandler for TimelinePanel<E> {
 			.update(cx, |engine, cx| engine.sync_clips_by_waveform(ids, true, cx));
 		true
 	}
+	/// 编辑 → 链接/重新链接: toggles the graph links among the selected
+	/// clips through the engine (one undoable entry).
+	fn toggle_links(&mut self, cx: &mut Context<Self>) -> bool {
+		let ids: Vec<ClipId> = self.timeline.read(cx).selection().iter().copied().collect();
+		self.engine
+			.update(cx, |engine, cx| engine.toggle_clip_links(ids, cx));
+		true
+	}
 
 	// --- view ---
 	fn zoom_in(&mut self, cx: &mut Context<Self>) -> bool {