Add header_rows: u32 field to GridCandidate struct to store the count
of contiguous header rows detected. This completes the output requirement
"Table.header_rows: u32" from the header row detection task.
The header row detection logic was already fully implemented in cell.rs:
- Bold font detection via PostScript name patterns
- Cell-level and row-level bold detection
- Combined header detection (bold OR TH signals)
- Multi-row header counting
- Cell header flag marking
This commit only adds the field to store the header count on the
GridCandidate struct and updates constructors.
Co-Authored-By: Claude Code <noreply@anthropic.com>
