Reduce preview composable invalidations for non-subgraph Vue nodes

[coderabbitai]

Summary

Reduce unnecessary reactive evaluations from preview composables in Vue graph nodes.

LGraphNode.vue currently instantiates both usePromotedPreviews and useAmbientSubgraphPreviews for every LGraphNode. For non-subgraph nodes, these composables short-circuit. However, preview output updates can still invalidate their computed state across all Vue nodes.

Rationale

Large workflows can contain 500 or more nodes but few subgraph nodes. A WebSocket preview frame can cause O(total nodes) short-circuit evaluations. The new ambient-preview path has the same subscription pattern as the existing promoted-preview path. Address both paths together in a dedicated performance change.

Affected areas

• src/renderer/extensions/vueNodes/components/LGraphNode.vue
• usePromotedPreviews
• useAmbientSubgraphPreviews
• Reactive dependencies on node output / preview state

Required changes

• Determine a design that prevents non-subgraph LGraphNode instances from subscribing to preview-output changes.
• Apply the design consistently to both promoted previews and ambient previews.
• Preserve preview behavior, ordering, and exposure precedence for SubgraphNode instances.
• Keep this work separate from pull request fix: decouple subgraph preview/output display from the promotion system #14569.

Acceptance criteria

• Non-subgraph Vue nodes do not perform preview-composable recomputation for unrelated preview frames.
• Subgraph nodes continue to render promoted and ambient previews correctly.
• An explicit exposure continues to take precedence over an ambient preview for the same source node.
• Add or update focused tests where the existing test structure supports this behavior.

Backlinks

• Source pull request: fix: decouple subgraph preview/output display from the promotion system #14569
• Source review comment: fix: decouple subgraph preview/output display from the promotion system #14569 (comment)
• Requested by: @christian-byrne