.ai file import fidelity, specifically handling of page background elements versus actual artwork layers.

**Version:** `main` at 0d4d125, `vectorcraft-cli` (headless), macOS 26.6, M2 Max.

### What happens

Opening a PDF-compatible `.ai` whose PDF part paints a white rectangle over the whole page **outside its optional-content layers**, and then the layers' art, gives the document an extra layer named "Page 1" holding a page-sized white path, under the file's own layers.

On real files saved by the reference app (version 30, one artboard, one layer), the PDF part starts with exactly such a fill (`/CS0 cs 1 1 1 scn 0 h w -h re f`, before the first `/OC /MC0 BDC`). The EPS saved from the same artwork has 15 paths and no background; the `.ai` opens with 16, the extra one being this fill.

Consequences:
- White artwork (a white logo variant) sits on an opaque white rectangle and looks empty on the canvas.
- `convert x.ai x.svg` / `x.png` / `x.pdf` all carry the opaque white background, so a logo exported for a dark web page gets a white box.

### Expected

The page fill isn't part of any layer, so it isn't artwork: the file opens with only its layers (as the EPS of the same artwork does), with a transparent background. Possibly limited to files with private data (`PieceInfo` / `AIPrivateData`) and layers (`OCProperties`), where content outside every layer is the app's page, not art.

### Repro (synthetic)

```python
# repro.ai: one layer, white and orange squares on it, a white page fill outside it.
content = b"""1 1 1 rg 0 0 200 100 re f
/OC /MC0 BDC
1 1 1 rg 20 20 60 60 re f
1 0.5 ...