feat(pdftract-40oz0): implement document-level fields for Phase 6.1
Add top-level Output struct with all document-level fields per Phase 6.1
spec (plan lines 2004-2014). Includes DocumentMetadata, OutlineNode,
PageJson, DiagnosticJson, and Phase 7 placeholder types (ThreadJson,
AttachmentJson, LinkJson, AnnotationJson).
All acceptance criteria PASS:
- Empty Output serializes with all 11 document-level keys
- Phase 7 placeholder fields present as empty arrays
- JSON Schema generation via schemars feature
- Round-trip serde test passes
Closes: pdftract-40oz0