fix(ext-api): add idempotency guard for v2 demo extensions per RFR-12144-1

Per workspace executive decision (option a) on F-12144-1: v1 + v2 conversions
in dynamicPrompts/imageCrop/previewAny coexist as Phase A demos following the
strangler-fig migration pattern (D6). Both register, but only one path runs
per node — guards skip v2 when v1 is already registered to prevent:

- dynamicPrompts: double processDynamicPrompt() on serialize
- previewAny: duplicate preview_markdown/preview_text/previewMode widgets
- imageCrop: redundant setSize call (idempotent but cleaner)

Guard pattern: 1-line useExtensionStore().isExtensionInstalled('<v1-name>') check
at the top of nodeCreated.

See workspace AGENTS.md rule #8 — CI failures on the ext-api stack do not
block flips during Phase A; full CI green required only at rebase point onto
PR #11939.

src/extensions/core/imageCrop.v2.ts

@@ -6,12 +6,17 @@
  */
 
 import { defineNode, type NodeHandle } from '@/extension-api'
+import { useExtensionStore } from '@/stores/extensionStore'
 
 defineNode({
   name: 'Comfy.ImageCrop.V2',
   nodeTypes: ['ImageCropV2'],
 
   nodeCreated(node: NodeHandle) {
+    // RFR-12144-1 strangler-fig guard (D6): no-op if v1 is registered so the
+    // legacy path owns sizing during Phase A coexistence.
+    if (useExtensionStore().isExtensionInstalled('Comfy.ImageCrop')) return
+
     const [w, h] = node.getSize()
     node.setSize([Math.max(w, 300), Math.max(h, 450)])
   }

src/extensions/core/previewAny.v2.ts

@@ -14,12 +14,19 @@ import {
   type NodeExecutedEvent,
   type WidgetValueChangeEvent
 } from '@/extension-api'
+import { useExtensionStore } from '@/stores/extensionStore'
 
 defineNode({
   name: 'Comfy.PreviewAny.V2',
   nodeTypes: ['PreviewAny'],
 
   nodeCreated(node: NodeHandle) {
+    // RFR-12144-1 strangler-fig guard (D6): v1 + v2 coexist as Phase A demos,
+    // but only one path runs per node. v1 adds the same three widgets via
+    // beforeRegisterNodeDef + onNodeCreated patching, so if v1 is registered
+    // we no-op to avoid duplicate widgets on the node.
+    if (useExtensionStore().isExtensionInstalled('Comfy.PreviewAny')) return
+
     const markdown = node.addWidget('MARKDOWN', 'preview_markdown', '', {
       hidden: true,
       readonly: true,