Add multi-output nodes with struct returns destructured by #[node_macro::destructure]

A #[node_macro::node] function returning a struct tagged with
field is a named output connector (title-cased from the field name,
renamed with #[name("...")], described by its doc comment). By default
the node has a hidden primary output carrying the whole struct with the
fields as secondary outputs; marking at most one field #[primary] makes
that field the primary output instead.

The macro generates one hidden extractor proto node per field plus a
registration keyed by the struct's TypeId. The Graphene preprocessor
recognizes nodes returning a registered struct and substitutes them, in
the transient runtime copy of the network only, with a generated network
exporting each field through its extractor. The destructuring machinery
therefore never appears when drilling into a node, in copied clipboard
content, or in saved documents. When a Memoize implementation is
registered for the struct type, the struct is computed once and shared
across all outputs rather than re-evaluated per output.

The editor derives output counts, names, and types for such nodes from
the registry. The old hand-authored "Split Vec2" and "Split Channels"
wrapper-network definitions are replaced by multi-output split_vec2 and
split_channels proto nodes, with document migrations that keep existing
wires valid since the output indices are unchanged.

The "Position on Path" and "Tangent on Path" nodes are combined into a
single multi-output "Evaluate Path" node whose primary output is the
position and whose secondary output is the tangent angle. A migration
converts old instances, forwarding the shared inputs and remapping the
tangent nodes' downstream connections to the new tangent output index.

The now-redundant "Extract XY" node is removed (its role is subsumed by
Split Vec2's destructuring), and "Extract Channel" becomes a plain helper
used by Split Channels rather than a standalone node.

Cargo.lock

@@ -4467,10 +4467,15 @@ checksum = "925383efa346730478fb4838dbe9137d2a47675ad789c546d150a6e1dd4ab31c"
 name = "preprocessor"
 version = "0.1.0"
 dependencies = [
+ "core-types",
+ "dyn-any",
+ "futures",
+ "glam",
  "graph-craft",
  "graphene-std",
  "interpreted-executor",
  "log",
+ "node-macro",
 ]
 
 [[package]]