Separate promoted-widget identity from instance value persistence

Follow-up from #11559 and DrJKL's review.

Current state:
- `src/lib/litegraph/src/subgraph/SubgraphNode.ts:1142` reads promoted-widget identity from `SubgraphNode.properties.proxyWidgets`.
- `src/lib/litegraph/src/subgraph/SubgraphNode.ts:1728` writes per-instance inline values back into the same `proxyWidgets` entries.

Goal:
- Move promotion identity toward the subgraph definition rather than each `SubgraphNode` instance.
- Persist per-instance promoted-widget values in a dedicated instance-state slot instead of mixing them into `properties.proxyWidgets`.
- Provide a migration path for existing tuple-based `proxyWidgets` entries.

Why:
- Promotion identity and per-instance values have different ownership and mutation cadence. Separating them should reduce save/load churn and make later migration safer.

┆Issue is synchronized with this [Notion page](https://app.notion.com/p/Issue-11740-Separate-promoted-widget-identity-from-instance-value-persistence-3516d73d365081bb9ba9f57c3739ef0c) by [Unito](https://www.unito.io)