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/annot.rs

@@ -292,7 +292,7 @@ fn pdf_text_string(s: &str) -> LoObj {
 /// Tìm file TTF có Unicode trên hệ thống theo đậm/nghiêng (Windows/Linux/Mac).
 /// Thử đúng file biến thể (vd `arialbd.ttf`) trước, rồi mới hạ cấp dần về regular
 /// của family khác — để FreeText giữ đúng đậm/nghiêng như Foxit.
-fn find_font_bytes(bold: bool, italic: bool) -> Option<Vec<u8>> {
+pub(crate) fn find_font_bytes(bold: bool, italic: bool) -> Option<Vec<u8>> {
     #[cfg(windows)]
     {
         // [regular, bold, italic, bold-italic] cho từng family phổ biến trên Windows.