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/nodes/mod.rs

@@ -36,7 +36,7 @@ mod mathbase;
 mod matrix;
 mod merge;
 mod mosaicfilternode;
-mod multicamnode;
+pub mod multicamnode;
 mod noise;
 mod ociobase;
 mod ociogradingtransformlinear;

crates/oaknode/src/nodes/multicamnode.rs

@@ -69,6 +69,28 @@ impl MultiCamNode {
 		core.standard_value(CURRENT_INPUT, -1).to_double() as i32
 	}
 
+	/// The connected sequence node (C++ `sequence_`).
+	pub fn sequence(&self) -> Option<NodeId> {
+		self.sequence
+	}
+
+	/// Set/clear the connected-sequence state (the effects of the C++
+	/// `InputConnectedEvent`/`InputDisconnectedEvent` on `sequence_in`:
+	/// store the sequence and toggle the `sequence_type_in` hidden flag).
+	/// The graph arena does not dispatch behavior events on edge edits, so
+	/// the commands that edit the `sequence_in` edge call this to keep the
+	/// behavior state in sync with the graph.
+	pub fn set_sequence(&mut self, core: &mut NodeCore, sequence: Option<NodeId>) {
+		if let Some(slot) = core.get_input_mut(SEQUENCE_TYPE_INPUT) {
+			if sequence.is_some() {
+				slot.flags &= !crate::input::flags::HIDDEN;
+			} else {
+				slot.flags |= crate::input::flags::HIDDEN;
+			}
+		}
+		self.sequence = sequence;
+	}
+
 	/// Number of available sources (C++ `get_source_count()`): the
 	/// connected sequence's track count when a sequence is set,
 	/// otherwise the `sources_in` array size.