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/node.rs

@@ -120,6 +120,10 @@ pub mod flags {
 	pub const AUDIO_EFFECT: u64 = 0x4;
 	/// `k_dont_show_in_create_menu`.
 	pub const DONT_SHOW_IN_CREATE_MENU: u64 = 0x8;
+	/// `k_is_item` (C++ `node.h:119`): bin items (folders, footage,
+	/// sequences) — dependency-graph copies share these instead of
+	/// cloning them (`// CPP-PARITY: node.cpp:1159,1199`).
+	pub const IS_ITEM: u64 = 0x10;
 }
 
 impl NodeCore {