Subgraph system: ~5,230 lines of skipped tests + 2 unimplemented methods

## Recommendation

### 🔧 FINISH: Restore subgraph test coverage and implement missing methods

The subgraph feature was migrated but test coverage was never restored and two key methods throw runtime errors.

## What Was Found

### Skipped Tests (~5,230 lines across 14 files)
Every subgraph test file in `src/lib/litegraph/src/subgraph/*.test.ts` begins with `// TODO: Fix these tests after migration`. ~70+ `describe.skip` blocks spanning serialization, events, memory management, edge cases, slot connections, widget promotion, conversion, visual feedback, and node construction.

### Unimplemented Methods (Runtime Crashes)
- `src/lib/litegraph/src/canvas/ToOutputFromIoNodeLink.ts:72` — `connectToSubgraphInput()` throws `'Not implemented'`
- `src/lib/litegraph/src/canvas/ToInputFromIoNodeLink.ts:77` — `connectToSubgraphOutput()` throws `'Not implemented'`

Users hitting these code paths (connecting links to subgraph boundary IO nodes) get a runtime crash.

## Evidence

- 14 test files with `describe.skip` blocks
- 2 `throw new Error('Not implemented')` in production code paths

## Context

The subgraph (nested workflow) feature was migrated but the test suite and some wiring methods were left incomplete.

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**Category:** Half-Implemented | **Confidence:** HIGH | **Tools:** Semantic analysis
_Filed by audit-code skill_

Part of #11022

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11032-Subgraph-system-5-230-lines-of-skipped-tests-2-unimplemented-methods-33e6d73d3650815590fee2cc7024b757) by [Unito](https://www.unito.io)