…st” error (anomalyco#200)

Resolves anomalyco/opentui#199

## The Bug

When inserting a React element in the middle of a list inside a
`<scrollbox>`, the application crashes with:

```
Error: Anchor does not exist
  at insertBefore (packages/core/src/Renderable.ts:1118:17)
  at insertBefore (packages/react/src/reconciler/host-config.ts:65:12)
```

## Root Cause

ScrollBox has an internal wrapper structure where user content is added
to a nested child (`this.content`), not directly to the ScrollBox
instance. The component hierarchy looks like:

```
ScrollBox
├── wrapper (scroll-box-wrapper)
│   └── viewport
│       └── content ← User children go here
└── scrollbar (scroll-box-vertical-scrollbar)
```

ScrollBox correctly overrides `add()` and `remove()` to delegate to
`this.content`:

- `add(obj, index)` → `this.content.add(obj, index)` ✓
- `remove(id)` → `this.content.remove(id)` ✓

**However, ScrollBox was missing an override for `insertBefore()`.**

When React inserts an element in the middle of a list (e.g., when a
conditional element appears after initial render), it calls:

```typescript
parent.insertBefore(newChild, anchorChild)
```

Without the override, this uses the base `Renderable.insertBefore()`
which:

1. Looks for `anchorChild` in the parent's direct children via
`renderableMapById`
2. Fails because `anchorChild` is actually in `this.content` (the
wrapper), not in ScrollBox's direct children

## The Fix

Added `insertBefore()` override in ScrollBox to delegate to
`this.content`, matching the pattern used for `add()` and `remove()`:

## Impact Analysis

### Affected Components

- ✅ **ScrollBox** - Fixed by adding `insertBefore()` override
- ✅ **TextRenderable** - Already had proper `insertBefore()` delegation

### Affected Renderers

- ❌ **React** - Was affected (uses `insertBefore()` directly)
- ✅ **Solid** - Not affected (uses `add(node, anchorIndex)` pattern)
- ✅ **Vue** - Not affected (uses `add(node, anchorIndex)` pattern)