refactor: migrate ES private fields to TypeScript private for Vue Proxy compatibility (#8440)

## Summary

Migrates ECMAScript private fields (`#`) to TypeScript private
(`private`) across LiteGraph to fix Vue Proxy reactivity
incompatibility.

## Problem

ES private fields (`#field`) are incompatible with Vue's Proxy-based
reactivity system - accessing `#field` through a Proxy throws
`TypeError: Cannot read private member from an object whose class did
not declare it`.

## Solution

- Converted all `#field` to `private _field` across 10 phases
- Added `toJSON()` methods to `LGraph`, `NodeSlot`, `NodeInputSlot`, and
`NodeOutputSlot` to prevent circular reference errors during
serialization (TypeScript private fields are visible to `JSON.stringify`
unlike true ES private fields)
- Made `DragAndScale.element.data` non-enumerable to break canvas
circular reference chain

## Testing

- All 4027 unit tests pass
- Added 9 new serialization tests to catch future circular reference
issues
- Browser tests (undo/redo, save workflows) verified working

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8440-refactor-migrate-ES-private-fields-to-TypeScript-private-for-Vue-Proxy-compatibility-2f76d73d365081a3bd82d429a3e0fcb7)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/scripts/api.ts

@@ -294,7 +294,7 @@ export class PromptExecutionError extends Error {
 }
 
 export class ComfyApi extends EventTarget {
-  #registered = new Set()
+  private _registered = new Set()
   api_host: string
   api_base: string
   /**
@@ -451,7 +451,7 @@ export class ComfyApi extends EventTarget {
   ) {
     // Type assertion: strictFunctionTypes.  So long as we emit events in a type-safe fashion, this is safe.
     super.addEventListener(type, callback as EventListener, options)
-    this.#registered.add(type)
+    this._registered.add(type)
   }
 
   override removeEventListener<TEvent extends keyof ApiEvents>(
@@ -492,7 +492,7 @@ export class ComfyApi extends EventTarget {
   /**
    * Poll status  for colab and other things that don't support websockets.
    */
-  #pollQueue() {
+  private _pollQueue() {
     setInterval(async () => {
       try {
         const resp = await this.fetchApi('/prompt')
@@ -568,7 +568,7 @@ export class ComfyApi extends EventTarget {
     this.socket.addEventListener('error', () => {
       if (this.socket) this.socket.close()
       if (!isReconnect && !opened) {
-        this.#pollQueue()
+        this._pollQueue()
       }
     })
 
@@ -691,7 +691,7 @@ export class ComfyApi extends EventTarget {
               )
               break
             default:
-              if (this.#registered.has(msg.type)) {
+              if (this._registered.has(msg.type)) {
                 // Fallback for custom types - calls super direct.
                 super.dispatchEvent(
                   new CustomEvent(msg.type, { detail: msg.data })
@@ -956,7 +956,7 @@ export class ComfyApi extends EventTarget {
    * @param {*} type The endpoint to post to
    * @param {*} body Optional POST data
    */
-  async #postItem(type: string, body?: Record<string, unknown>) {
+  private async _postItem(type: string, body?: Record<string, unknown>) {
     try {
       await this.fetchApi('/' + type, {
         method: 'POST',
@@ -976,7 +976,7 @@ export class ComfyApi extends EventTarget {
    * @param {number} id The id of the item to delete
    */
   async deleteItem(type: string, id: string) {
-    await this.#postItem(type, { delete: [id] })
+    await this._postItem(type, { delete: [id] })
   }
 
   /**
@@ -984,7 +984,7 @@ export class ComfyApi extends EventTarget {
    * @param {string} type The type of list to clear, queue or history
    */
   async clearItems(type: string) {
-    await this.#postItem(type, { clear: true })
+    await this._postItem(type, { clear: true })
   }
 
   /**
@@ -993,7 +993,7 @@ export class ComfyApi extends EventTarget {
    * @param {string | null} [runningPromptId] Optional Running Prompt ID to interrupt
    */
   async interrupt(runningPromptId: string | null) {
-    await this.#postItem(
+    await this._postItem(
       'interrupt',
       runningPromptId ? { prompt_id: runningPromptId } : undefined
     )