[RFC] Consolidate Load3D domain — 19 files across 5 directories

## Problem

Load3D is a self-contained feature domain scattered across 5 directories:

- `src/composables/useLoad3d.ts`, `useLoad3dDrag.ts`, `useLoad3dViewer.ts`
- `src/components/load3d/` (multiple Vue components)
- `src/extensions/core/load3d/` (extension registration + utils)
- `src/extensions/core/load3d.ts`, `load3dLazy.ts`
- `src/services/load3dService.ts`

Working on Load3D requires navigating 5 directory trees.

## Proposed Fix

Consolidate into `platform/load3d/`:

```
platform/load3d/
composables/useLoad3d.ts
composables/useLoad3dDrag.ts
composables/useLoad3dViewer.ts
components/Load3dViewer.vue
services/load3dService.ts
extension.ts
```

## Migration Plan

- Single PR to relocate all files
- Update imports

## Testing Strategy

- All existing Load3D tests pass
- `pnpm typecheck` passes

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11084-RFC-Consolidate-Load3D-domain-19-files-across-5-directories-33e6d73d365081919df3e1144b20330b) by [Unito](https://www.unito.io)