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/commands/graphCommands.ts

@@ -6,7 +6,9 @@ import {
   type GraphNode,
   portsCompatible,
 } from '../graph/Graph';
+import { isTerminalType, resolveDef, terminalNode } from '../graph/nested';
 import { nodeDef } from '../graph/NodeRegistry';
+import { behaviorLookup } from '../model/behaviorPrograms';
 import { newId } from '../ids';
 import type { PropertyBag, PropertyValue } from '../model/Properties';
 import type { Project } from '../model/Project';
@@ -42,7 +44,12 @@ export class AddNodeCommand implements Command {
 
   execute(project: Project): void {
     requireNodeDef(this.node.type);
-    requireBehavior(project, this.assetId).graph.nodes.push(this.node);
+    const graph = requireBehavior(project, this.assetId).graph;
+    // The two terminals give the behavior its signature; a second one of either would make
+    // "which ports does this behavior have" ambiguous.
+    if (isTerminalType(this.node.type) && terminalNode(graph, this.node.type))
+      throw new CommandError(`This behavior already has a ${nodeDef(this.node.type)?.label} node`);
+    graph.nodes.push(this.node);
   }
 
   undo(project: Project): void {
@@ -153,8 +160,9 @@ export class ConnectCommand implements Command {
     const toNode = findNode(graph, this.edge.to.node);
     if (!fromNode || !toNode) throw new CommandError('Node not found');
     if (fromNode === toNode) throw new CommandError('A node cannot connect to itself');
-    const out = requireNodeDef(fromNode.type).outputs.find((p) => p.key === this.edge.from.port);
-    const inp = requireNodeDef(toNode.type).inputs.find((p) => p.key === this.edge.to.port);
+    const lookup = behaviorLookup(project);
+    const out = resolveDef(fromNode, lookup)?.outputs.find((p) => p.key === this.edge.from.port);
+    const inp = resolveDef(toNode, lookup)?.inputs.find((p) => p.key === this.edge.to.port);
     if (!out || !inp) throw new CommandError('Port not found');
     if (!portsCompatible(out.type, inp.type))
       throw new CommandError(`Cannot connect ${out.type} to ${inp.type}`);