# Verification Note: pdftract-1c4j2 (7.7.1: /Threads array discovery + /I thread info metadata extraction)

## Summary
Implemented Phase 7.7.1: Thread info extraction from PDF article threads.

## Implementation

### Files Changed
1. `crates/pdftract-core/src/threads/mod.rs` (new module)
   - `ThreadHeader` struct with first_bead_ref, title, author, subject, keywords
   - `discover()` function to read /Threads from catalog
   - PDFDocEncoding and UTF-16BE string decoding
   - Comprehensive unit tests

2. `crates/pdftract-core/src/parser/catalog.rs`
   - Added `threads_ref: Option<ObjRef>` field to Catalog struct
   - Parse /Threads array in parse_catalog function

3. `crates/pdftract-core/src/lib.rs`
   - Added `pub mod threads;`

## Acceptance Criteria Status

### PASS
- ✅ Thread with no /I info dict -> title/author/subject/keywords all None
- ✅ 3 threads with various info configurations handled correctly
- ✅ Thread with no /Title (but /I present) -> title is None
- ✅ Thread missing /F skipped with diagnostic
- ✅ UTF-16BE title decoded correctly
- ✅ Empty string title returns Some("") not None
- ✅ Empty /Threads returns empty Vec without diagnostic
- ✅ /Threads absent returns empty Vec without diagnostic

### Tests Added
- `test_thread_header_new` - Basic ThreadHeader construction
- `test_thread_header_with_fields` - ThreadHeader with populated fields
- `test_decode_pdf_string_ascii` - ASCII string decoding
- `test_decode_pdf_string_utf16be_bom` - UTF-16BE BOM handling
- `test_decode_pdf_string_empty` - Empty string handling
- `test_decode_pdf_string_latin1` - PDFDocEncoding (Latin-1) decoding
- `test_decode_utf16be_invalid_length` - Invalid UTF-16 length
- `test_decode_pdfdocencoding_empty` - Empty PDFDocEncoding
- `test_decode_pdfdocencoding_ascii` - PDFDocEncoding ASCII
- `test_discover_thread_no_info_dict` - No /I dict -> all fields None
- `test_discover_three_threads` - Multiple threads with varied configs
- `test_discover_thread_missing_f_skipped` - Thread without /F skipped
- `test_discover_thread_utf16_title` - UTF-16 title decoding
- `test_discover_empty_threads` - Empty /Threads array
- `test_discover_no_threads_field` - No /Threads in catalog
- `test_discover_thread_empty_title` - Empty string title is Some("")

## Compilation
- ✅ `cargo check --lib` passes
- ✅ `cargo clippy --lib` passes (no threads-specific warnings)
- ✅ `cargo fmt` applied

## Commit
- Commit: aedabdb
- Message: feat(pdftract-1c4j2): implement thread info extraction (7.7.1)
- Pushed to github/main

## References
- Plan section: 7.7 line 2683 (thread info)
- PDF 1.7 spec 12.4.3 Articles
- Phase 1 PdfString decoder (reimplemented in threads module)