doc: Apple textutil-authored .doc is rejected at open — cfb reports "Malformed MiniFAT (mini sector 0 pointed to twice)"

## Summary

`.doc` files authored by Apple's Cocoa exporter (TextEdit "Save as… Word 97", `textutil -convert doc`) are rejected at open:

```
malformed document: not an OLE2 compound file: Malformed MiniFAT (mini sector 0 pointed to twice)
```

Word-authored `.doc` files convert fine, including this repo's fixtures. So this only matters where macOS-authored `.doc` uploads occur (in our pipeline: e-mail attachments written by TextEdit).

## Reproduction

On any Mac (this one: macOS 26.2, build 25C56):

```
printf 'Alpha paragraph.\n\nBeta paragraph.\n' | textutil -stdin -convert doc -output textedit-minimal.doc
```

Converting `textedit-minimal.doc` at v0.1.6 / HEAD `8eecca5` (Node binding and `cargo run --example convert` agree) yields the error above.

Sample (19 KB, generated exactly as shown): [textedit-minimal.doc](https://github.com/H0rowitz/anydoc/raw/apple-doc-sample/issue-samples/textedit-minimal.doc)

## Analysis, as far as we took it

anydoc already opens OLE2 containers with the `cfb` crate's *lenient* mode (`CompoundFile::open`), so this is a shape Apple's writer emits that even lenient mode refuses — the pointwise fix probably lives in the `cfb` crate (or in a pre-repair pass on anydoc's side). Filing here first since it presents as an anydoc capability boundary and you may prefer to route or track it your own way; we have not diagnosed which MiniFAT invariant the Apple writer actually violates (the message suggests a mini-sector chain that self-references or is doubly linked). Happy to dig further or file it against `cfb` directly if that's more useful.

Low urgency from our side — flagging it mostly so the boundary is a known, recorded one rather than a surprise.

Reproduce the failure with the provided textutil command and issue-samples/textedit-minimal.doc. Start at the cfb CompoundFile::open path used by anydoc, then compare the cargo run --example convert and Node binding behavior. Done means establishing a supported handling path for this sample or documenting and routing the MiniFAT incompatibility to cfb.

Written by the indexing model from the issue text.