Add a core FILE_3D / uploaded GLB to MESH loader for mesh post-processing workflows

### Summary

Please add a core, API-executable conversion/loading path from an uploaded 3D file (`FILE_3D`, especially GLB/GLTF) to the in-memory `MESH` type used by the native mesh post-processing nodes.

### Current limitation

Core supports both sides independently:

- `Load3D` can select/upload GLB, GLTF, OBJ, FBX, and STL files.
- Native post-processing nodes such as `DecimateMesh`, `RemeshMesh`, `WeldVertices`, and `FillHoles` accept `MESH`.
- Saving nodes convert/output mesh data as `FILE_3D`.

However, the current node inventory does not expose a corresponding `FILE_3D → MESH` bridge. This prevents an existing uploaded or previously generated GLB from being passed into `DecimateMesh` in a standalone API workflow.

The limitation is especially visible on Comfy Cloud: an application can upload and preview/download a persisted GLB, and Cloud exposes `DecimateMesh`, but cannot construct a valid graph connecting that uploaded asset to the node.

### Desired workflow

```
Upload/Load GLB (FILE_3D)
→ Load/convert to MESH
→ DecimateMesh / RemeshMesh / WeldVertices / FillHoles
→ SaveGLB
```

The loader should be usable through prompt/API execution without depending on interactive frontend-only preview state.

### Suggested behavior

- Accept core-supported mesh file types, at minimum GLB/GLTF.
- Return `MESH` compatible with the existing native post-processing nodes.
- Preserve vertex colors where supported.
- Clearly define behavior or warnings for UVs, materials, textures, skins, morph targets, and animations that cannot survive conversion.
- Work with files uploaded into the normal Comfy input area and in Comfy Cloud.
- Produce actionable errors for missing files and unsupported formats.

### Why this matters

Without this bridge, native mesh processing works when a generator produces `MESH` within the same graph, but not for persisted assets or independent post-processing jobs. Applications must perform browser-side geometry processing or depend on custom nodes, which prevents portable local/Cloud workflows.

Start by tracing the existing Load3D path and the native DecimateMesh, RemeshMesh, WeldVertices, FillHoles, and SaveGLB nodes, focusing on their FILE_3D and MESH inputs and outputs. Define the API-executable conversion behavior, supported formats, preservation or warning rules, and errors for missing or unsupported files; done means uploaded or generated GLB/GLTF assets can be processed through this workflow locally and in Comfy Cloud.

Written by the indexing model from the issue text.