feat: resolveVirtualOutput for cross-subgraph virtual nodes (eg. Set/Get) (#10111)

## Summary

Enable virtual nodes (e.g. Set/Get) to resolve their output source
directly when the source lives in a different subgraph.

## Changes

- **What**: Added optional resolveVirtualOutput method on LGraphNode and
a new resolution path in ExecutableNodeDTO.resolveOutput that checks it
before falling through to the existing getInputLink path. Includes unit
tests for the three code paths (happy path, missing DTO, fallthrough).

## Review Focus

- Fully backwards compatible — no existing node implements
resolveVirtualOutput, so the new path is always skipped for current
virtual nodes (Reroute, PrimitiveNode, etc.).

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots

Simple example in actual use, combined with new changes in KJNodes
allows using Get nodes inside subgraphs:

<img width="2242" height="1434" alt="image"
src="https://github.com/user-attachments/assets/cc940a95-e0bb-4adf-91b6-9adc43a74aa2"
/>

<img width="1436" height="440" alt="image"
src="https://github.com/user-attachments/assets/62044af5-0d6e-4c4e-b34c-d33e85f2b969"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10111-feat-resolveVirtualOutput-for-cross-subgraph-virtual-nodes-eg-Set-Get-3256d73d3650816a9f20e28029561c58)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LGraphNode.ts

@@ -1207,6 +1207,14 @@ export class LGraphNode
       : this.inputs[slot]
   }
 
+  /**
+   * Resolves the output source for cross-graph virtual nodes (e.g. Set/Get),
+   * bypassing {@link getInputLink} when the source lives in a different graph.
+   */
+  resolveVirtualOutput?(
+    slot: number
+  ): { node: LGraphNode; slot: number } | undefined
+
   /**
    * Returns the link info in the connection of an input slot
    * @returns object or null