Remove deprecated LGraphCanvas panel properties (node_panel, options_panel, etc.)

## Recommendation

### 🗑️ SAFE TO REMOVE (after ecosystem check)

4 deprecated panel properties remain on `LGraphCanvas`:
- `node_panel`
- `options_panel`
- `SELECTED_NODE`
- `NODEPANEL_IS_OPEN`

Located at `src/lib/litegraph/src/LGraphCanvas.ts:729-744`, all marked `@deprecated Panels`.

## What Was Found

The panel system was migrated to Vue components but these legacy properties remain. They may be accessed by custom node extensions in the ecosystem.

## How to Fix

1. Search the custom node ecosystem for usage of these properties
2. If widely used: add deprecation warnings via `defineDeprecatedProperty`
3. If not used: remove the properties

## ⚠️ Extension Ecosystem Impact

Per ADR 0003: changes to entity properties affect 40+ custom node repos. Check before removing.

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**Category:** Vestigial | **Priority:** Nice-to-have | **Effort:** Small
Part of #11022
_Filed by repo-audit skill_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11083-Remove-deprecated-LGraphCanvas-panel-properties-node_panel-options_panel-etc-33e6d73d36508132b8f6f06cef8de90c) by [Unito](https://www.unito.io)