Phase 3 (Organize + robust save) + Phase 4 (Edit content); update docs

Engine:
- organize.rs: build_document(plan) core — insert/delete/rotate/crop/extract/merge/split
- watermark.rs: watermark + header/footer via PdfPages::watermark, CID Unicode fonts
- qpdf.rs: QPDF CLI wrapper — repair corrupt files, re-encrypt after edits, ensure_openable
- edit.rs: MOAT — list_objects + apply_edits (SetText VN, Transform, Delete, Add/ReplaceImage)
- Tests: organize/watermark/qpdf_safety/edit roundtrip — 43/43 engine tests green

UI (Tauri): "Tổ chức trang" mode (grid + drag-reorder + multi-select), "Sửa nội dung"
mode (WYSIWYG object overlay, edit-in-place), global undo/redo, robust file open.

Corpus: add encrypted.pdf + corrupt-truncated.pdf fixtures.

Docs: mark Phase 1-4 done in roadmap; document QPDF setup + engine module layout in
05-dev-setup; add 10-13 phase summaries & user-test checklists; README status.

crates/ff-engine/src/lib.rs

@@ -5,16 +5,27 @@
 //! ocr... sẽ thêm dần ở các phase sau (xem docs/03-roadmap.md, 04-architecture.md).
 
 pub mod annot;
+pub mod edit;
 pub mod meta;
+pub mod organize;
+pub mod qpdf;
 pub mod render;
 pub mod text;
+pub mod watermark;
 
 pub use annot::{
     apply_annotations, count_annotations, list_annotations, AnnotInfo, AnnotKind, AnnotSpec,
 };
+pub use edit::{apply_edits, list_objects, EditOp, ObjectInfo, ObjectKind};
 pub use meta::{outline, page_dims, OutlineItem, PageDim};
+pub use organize::{
+    build_document, delete_pages, extract_pages, identity_plan, merge_files, rotate_pages,
+    split_by_page_count, PagePlanEntry, PageSource,
+};
+pub use qpdf::{encrypt_with_password, ensure_openable, find_qpdf, repair};
 pub use render::{bind_pdfium, page_count, render_page_png, PageImage};
 pub use text::{extract_text, page_char_boxes, search, CharBox, Rect, SearchHit};
+pub use watermark::{add_header_footer, add_watermark, Anchor, HeaderFooterSpec, WatermarkSpec};
 
 /// Lỗi cấp engine.
 #[derive(Debug, thiserror::Error)]