Let a behavior be a node in another graph

A behavior that has a Start and an End node can now be dropped into any
other graph as a node of its own. What is declared on the start node
becomes the inputs of that node, what hangs on the end node its outputs;
the port list is derived from the sub graph, not copied, so declaring a
port in one graph grows the node in the other without touching it. The
node carries the behavior's name and the palette lists behaviors by
name, so a call is one grab instead of a node plus a dropdown.

An embedded behavior has no object and therefore no place for
variables: every call starts on the asset defaults and communicates
only through its ports. That is the conservative direction - state can
still be added later, and shared state already has a home in a behavior
object in the scene. Node ids are unique per graph only, so each call
gets its own pulse cache; without that, two graphs would collide.

Renaming a port used to silently drop the value typed on the calling
node. It is carried along now, project wide, like the edges.

src/core/model/behaviorPrograms.ts

@@ -0,0 +1,15 @@
+import type { BehaviorLookup } from '../graph/nested';
+import { resolveAssetProps } from '../properties/resolve';
+import { findAsset, type Project } from './Project';
+
+/**
+ * What a `Run behavior` node needs from the project. `defaults` are the asset's variable values;
+ * every call copies them, so a nested behavior never carries state from one call to the next.
+ */
+export function behaviorLookup(project: Project): BehaviorLookup {
+  return (assetId) => {
+    const asset = findAsset(project, assetId);
+    if (!asset || asset.type !== 'behavior') return null;
+    return { name: asset.name, graph: asset.graph, defaults: resolveAssetProps(asset) };
+  };
+}