Implements 7.5.2: Filespec dictionary and EF stream decoder for PDF
embedded file attachments. Extracts filename (/UF preferred over /F),
description, MIME type, size, dates, and MD5 checksum from Filespec
dictionaries and decodes the embedded stream data.
Key additions:
- AttachmentBuilder struct with all attachment metadata fields
- extract_one() function for resolving Filespec and decoding EF stream
- PDF string decoding (UTF-16BE BOM, UTF-16BE without BOM, PDFDocEncoding)
- PDF date to ISO 8601 parsing (reused from signature module)
- 50 MB size limit enforcement with truncation flag
- Support for all Phase 1 stream filters (FlateDecode, LZWDecode, etc.)
Closes: pdftract-3lir
bd91f7d842