Let a node work out its own ports

Ports that vary per node already existed, but only for the three nested-behavior
types, and the rule for them sat as a switch on type names in nested.ts – the
one place in the project that knew node types by name. It dissolves into two
optional contracts on NodeDef, so a type carries its own rule:

  declaresPorts  the node holds an editable list, shown as a section in the
                 inspector – behavior start/end, and now Pack/Expand
  derivePorts    the node computes them from its settings – Sequence from a
                 count, Run behavior from the behavior it names

Sequence gains an Outputs setting (2 to 26). Its keys stay a, b, c and count on
with d, e, so the edges of the sequences already out there survive.

A param can now take a port away, and the edge on it would stay: it draws as
nothing yet survives every save, and comes back to life if the port returns.
SetNodeParamCommand takes those edges with it, which also closes the same leak
that switching a Run behavior node's target has always had.

The port editor addresses its node by id rather than by type, so two Pack nodes
can live in one graph, and it calls the rows Fields when they are not a
behavior's signature.

src/core/graph/nested.ts

@@ -1,5 +1,6 @@
-import { type PropertyBag, textOf } from '../model/Properties';
+import { textOf } from '../model/Properties';
 import { type DeclaredPort, declaredPortType, type Graph, type GraphNode } from './Graph';
+import type { BehaviorLookup } from './lookup';
 import { nodeDef, type NodeDef, type PortDef } from './NodeRegistry';
 
 export const BEHAVIOR_START = 'BehaviorStart';
@@ -8,17 +9,6 @@ export const RUN_BEHAVIOR = 'RunBehavior';
 
 export const BEHAVIOR_PARAM = 'behavior';
 
-/** What running another behavior's graph in place needs: its nodes, its variable defaults, its name. */
-export interface BehaviorProgram {
-  name: string;
-  graph: Graph;
-  defaults: PropertyBag;
-}
-
-export type BehaviorLookup = (assetId: string) => BehaviorProgram | null;
-
-export const NO_BEHAVIORS: BehaviorLookup = () => null;
-
 /** Identifier-like, and not `exec`: that key belongs to the fixed flow port of every terminal node. */
 export function isValidPortName(name: string): boolean {
   return /^[A-Za-z_][A-Za-z0-9_]*$/.test(name) && name !== 'exec';
@@ -37,34 +27,31 @@ export function isTerminalNode(node: GraphNode): boolean {
   return isTerminalType(node.type);
 }
 
-export function portsOf(port: DeclaredPort): PortDef {
+export function portDefOf(port: DeclaredPort): PortDef {
   return { key: port.key, label: port.key, type: declaredPortType(port.kind) };
 }
 
-function declaredOf(graph: Graph | undefined, type: string): PortDef[] {
+/** The ports the start (or end) node of `graph` declares, as seen from a node calling it. */
+export function declaredPorts(graph: Graph | undefined, type: string): PortDef[] {
   const node = graph && terminalNode(graph, type);
-  return (node?.ports ?? []).map(portsOf);
+  return (node?.ports ?? []).map(portDefOf);
 }
 
 /**
- * The node definition as it applies to *this* node. Identical to the registered one for every fixed
- * type; the three nested-behavior types grow ports from a declaration instead – the start and end
- * node from their own, the call node from those of the behavior it names.
+ * The node definition as it applies to *this* node: identical to the registered one, unless the
+ * type declares ports per node (`declaresPorts`) or computes them (`derivePorts`).
  */
 export function resolveDef(node: GraphNode, lookup: BehaviorLookup): NodeDef | undefined {
   const def = nodeDef(node.type);
-  if (!def) return undefined;
-  const own = (node.ports ?? []).map(portsOf);
-  if (node.type === BEHAVIOR_START) return { ...def, outputs: [...def.outputs, ...own] };
-  if (node.type === BEHAVIOR_END) return { ...def, inputs: [...def.inputs, ...own] };
-  if (node.type !== RUN_BEHAVIOR) return def;
-  const program = lookup(String(node.params[BEHAVIOR_PARAM] ?? ''));
+  if (!def || (!def.declaresPorts && !def.derivePorts)) return def;
+  const own = def.declaresPorts ? (node.ports ?? []).map(portDefOf) : [];
+  const side = (which: 'inputs' | 'outputs') => (def.declaresPorts === which ? own : []);
+  const grown = def.derivePorts?.({ params: node.params, behaviors: lookup }) ?? {};
   return {
     ...def,
-    // The graph reads as what it does, not as a row of identical "Run behavior" boxes.
-    label: program ? program.name : def.label,
-    inputs: [...def.inputs, ...declaredOf(program?.graph, BEHAVIOR_START)],
-    outputs: [...def.outputs, ...declaredOf(program?.graph, BEHAVIOR_END)],
+    label: grown.label ?? def.label,
+    inputs: [...def.inputs, ...side('inputs'), ...(grown.inputs ?? [])],
+    outputs: [...def.outputs, ...side('outputs'), ...(grown.outputs ?? [])],
   };
 }