Separate promoted-widget identity from instance value persistence

[dante01yoon]

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.

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