Implement Phase 7.3.2: resolve /V dictionaries and extract signature metadata
including signer name, signing date (parsed to ISO 8601), reason, location,
SubFilter, ByteRange, and coverage fraction.
Key changes:
- Add Signature struct with all metadata fields
- Add parse_pdf_date() for PDF date format to ISO 8601 conversion
- Add decode_pdf_string() for PDFDocEncoding/UTF-16BE string decoding
- Add extract_signature_metadata() and extract_signatures() public APIs
- Add 18 new unit tests (27 total tests, all PASS)
Acceptance criteria:
- Two signature fields: both extracted with correct signer names and dates
- Unsigned signature field: emitted with empty fields (value: null analog)
- /ByteRange coverage: correctly computed as fraction of file bytes
- Malformed date: returns None; missing /Name: returns ""; missing /ByteRange: returns None
Closes: pdftract-6arz
9992eb98d4