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/graph/Graph.ts

@@ -18,6 +18,24 @@ export interface GraphNode {
   x: number;
   y: number;
   params: PropertyBag;
+  /**
+   * Ports the node declares itself, on top of the fixed ones of its type. Only the behavior
+   * start/end nodes carry this – they are what gives a nested behavior its signature.
+   */
+  ports?: DeclaredPort[];
+}
+
+/** A port a start/end node declares. `kind` maps to the PortType of the same name (`text` → string). */
+export interface DeclaredPort {
+  key: string;
+  kind: DeclaredPortKind;
+}
+
+export const DECLARED_PORT_KINDS = ['number', 'boolean', 'text', 'object'] as const;
+export type DeclaredPortKind = (typeof DECLARED_PORT_KINDS)[number];
+
+export function declaredPortType(kind: DeclaredPortKind): PortType {
+  return kind === 'text' ? 'string' : kind;
 }
 
 export interface GraphEdge {