Implements Phase 7.3.1: AcroForm signature field discovery.
Walks /Fields array recursively, filters to /FT /Sig fields,
and extracts full_name, v_ref, rect, page_index, field_ref.
- Created signature module at crates/pdftract-core/src/signature/mod.rs
- Implemented walk_acroform_fields helper for reuse by 7.4
- Implemented sig::discover public API
- Added SigFieldRef struct with all required fields
- Handled /FT inheritance from parent fields
- Constructed absolute field names via dot-joined /T values
- Added comprehensive unit tests (9 tests, all passing)
Acceptance criteria:
- Discovery returns all /FT /Sig fields, including nested ones
- Unit tests: flat 2 sigs, nested 1 sig, no AcroForm, no Fields, /FT inheritance
- Public sig::discover(&Catalog) -> Vec<SigFieldRef>
- Reusable walk_acroform_fields helper available
Closes: pdftract-2wyd
fe15c81ba8