Behavior lifecycle: On enable, On disable and On destroy events (#42)

Order per instance: On start -> On enable -> On update ... -> On disable
-> On destroy. On enable fires right after On start and whenever the
behavior comes back on (itself and its parents); On disable in the frame
it goes off, the instance stays frozen. On destroy fires when the entity
leaves the world (Destroy node, child of a destroyed object) and when the
world is torn down on stop/restart, preceded by On disable if still on.
A new On start node added while playing still does not fire for a
running instance (kept as is, documented in the wiki).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/graph/nodes/events/OnDestroy.ts

@@ -0,0 +1,12 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { exec } from '../shared';
+
+export const OnDestroy: NodeDef = {
+  type: 'OnDestroy',
+  label: 'On destroy',
+  category: 'event',
+  description: 'Runs once when the object is destroyed – also when the game stops or restarts.',
+  inputs: [],
+  outputs: [exec()],
+  params: [],
+};

src/core/graph/nodes/events/OnDisable.ts

@@ -0,0 +1,12 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { exec } from '../shared';
+
+export const OnDisable: NodeDef = {
+  type: 'OnDisable',
+  label: 'On disable',
+  category: 'event',
+  description: 'Runs when the behavior is switched off (itself or a parent), and before On destroy.',
+  inputs: [],
+  outputs: [exec()],
+  params: [],
+};

src/core/graph/nodes/events/OnEnable.ts

@@ -0,0 +1,12 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { exec } from '../shared';
+
+export const OnEnable: NodeDef = {
+  type: 'OnEnable',
+  label: 'On enable',
+  category: 'event',
+  description: 'Runs right after On start, and again each time the behavior is switched back on.',
+  inputs: [],
+  outputs: [exec()],
+  params: [],
+};

src/core/graph/nodes/index.ts

@@ -71,6 +71,9 @@ import { OnClick } from './events/OnClick';
 import { OnCollisionEnter } from './events/OnCollisionEnter';
 import { OnCollisionExit } from './events/OnCollisionExit';
 import { OnCollisionStay } from './events/OnCollisionStay';
+import { OnDestroy } from './events/OnDestroy';
+import { OnDisable } from './events/OnDisable';
+import { OnEnable } from './events/OnEnable';
 import { OnKeyDown } from './events/OnKeyDown';
 import { OnKeyHeld } from './events/OnKeyHeld';
 import { OnKeyUp } from './events/OnKeyUp';
@@ -94,7 +97,10 @@ import { Variables } from './variable/Variables';
  */
 export const BUILT_IN_NODES: readonly NodeDef[] = [
   OnStart,
+  OnEnable,
   OnUpdate,
+  OnDisable,
+  OnDestroy,
   OnKeyDown,
   OnKeyUp,
   OnKeyHeld,