Implement or remove subgraph IO link connection stubs (throw 'Not implemented')

## Recommendation

### 🔧 FINISH: Complete Subgraph IO Link Connections

Two methods explicitly throw `'Not implemented'`:

- `connectToSubgraphOutput()` in `src/lib/litegraph/src/canvas/ToInputFromIoNodeLink.ts:84`
- `connectToSubgraphInput()` in `src/lib/litegraph/src/canvas/ToOutputFromIoNodeLink.ts:72`

## What Was Found

These are part of the subgraph IO node link system. The methods exist in the class hierarchy but throw at runtime if called. This could cause unexpected errors for users working with subgraphs.

## How to Fix

Either:
1. **Implement** the subgraph IO node link connections (preferred if the feature is needed)
2. **Remove** if the code paths are unreachable and the feature was abandoned

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**Category:** Half-Implemented | **Priority:** Recommended | **Effort:** Medium
Part of #11022
_Filed by repo-audit skill_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11069-Implement-or-remove-subgraph-IO-link-connection-stubs-throw-Not-implemented-33e6d73d3650814e93bac492277bb379) by [Unito](https://www.unito.io)