feat(pdftract-core): add /AF associated files array walker
Implements pdftract-zl9y3: PDF 2.0 /AF (Associated Files) array walker.
- Created attachment module with associated_files.rs
- walk_af_array() extracts /AF array from document catalog
- AssociatedFileEntry holds optional /AFRelationship and filespec_ref
- Returns empty Vec for PDF 1.7 documents (no /AF key)
- Supports all 6 PDF 2.0 relationship types: Source, Data, Alternative,
Supplement, EncryptedPayload, Unspecified
All 12 unit tests pass. Gates: check ✓ clippy ✓ fmt ✓ tests ✓
Closes: pdftract-zl9y3
027d3b4ee4