feat(pdftract-5w6i): implement AcroForm field walker with recursive walk and dot-joined names
Implements Phase 7.4.1: AcroForm field walker (recursive /Fields + dot-joined names).
## Changes
- Create `crates/pdftract-core/src/forms/mod.rs` module with:
- `AcroFieldType` enum (Tx, Btn, Ch, Sig, Other)
- `AcroFormField` struct with full field metadata
- `walk_acroform_fields()` public API function
- Recursive DFS traversal with /FT, /V, /DV, /Ff inheritance
- Widget annotation to page index resolution
- Cycle detection via visited set
- Name collision handling (keep last, emit diagnostic)
- Choice field option extraction for Ch fields
- Update `lib.rs` to export forms module and types
## Implementation Details
- Entry point: `/Catalog /AcroForm /Fields` array
- Dot-joined names: Concatenate `/T` values with "." separator
- Inheritance: `/FT`, `/V`, `/DV`, `/Ff` from parent to child
- Page resolution: Search page `/Annots` arrays for widget annotations
- Cycle detection: `visited` HashSet prevents infinite loops on malformed PDFs
- Name collisions: Track emitted names, keep last on duplicate
## Tests
All 15 unit tests pass:
- Flat 3 fields extraction
- Nested 2-level hierarchy with dot-joined names
- /FT inheritance from parent to child
- /FT override by child
- /Ff (flags) inheritance
- Empty /T segment handling
- Choice field /Opt array parsing
- All field types (Tx, Btn, Ch, Sig)
- Flag accessor methods (is_read_only, is_required, etc.)
- Button field is_checked() method
Closes: pdftract-5w6i
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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