feat(oaktimeline): multicam enable/disable/switch commands, split copies the dependency graph

- oaktimeline::multicam: clip_find_multicam (buffer/tex_in depth-1
  lookup), multicam_enable/disable (rewire sequence<->clip through a
  MultiCamNode), multicam_switch (split-preserving-links at the
  playhead, each half owns an independent multicam copy, linked clips
  switched together) as single undo commands with C++ labels.
- BlockSplitCommand now duplicates the clip's whole dependency graph
  (copy_node_and_dependency_graph_minus_items) instead of just the
  block core, matching the C++ BlockSplitCommand::prepare semantics;
  undo detaches the copied subgraph, redo re-attaches identity-
  preserving.
- oaknode: fix serializer dropping edges from the first-created node
  (ptr=0 was not registered in id_map), restoring sequence_in edge
  round-trips; multicam node and clip wiring serializer round-trip
  tests.

crates/oaknode/src/sequence.rs

@@ -77,6 +77,9 @@ impl SequenceBehavior {
 	/// from a file — the track lists arrive as separate nodes.
 	pub fn create() -> (NodeCore, Box<dyn NodeBehavior>) {
 		let mut core = NodeCore::new();
+		// Bin item (C++ `sequence.cpp:37` `set_flag(k_is_item)`): nested
+		// sequences are shared, never cloned, by dependency-graph copies.
+		core.flags |= crate::node::flags::IS_ITEM;
 		// Viewer parameter streams (C++ ViewerOutput::kVideoParamsInput /
 		// kAudioParamsInput / kSubtitleParamsInput arrays).
 		for (id, ty) in [