Initial commit: FoFreeXit Phase 1 (Viewer) + Phase 2 (Annotate)

PDF viewer (render/search/text-select/thumbnails/outline) and annotation
engine (Highlight/Underline/Strikeout/Square/FreeText/Note) over PDFium +
lopdf, with Unicode font embedding for Vietnamese text and undo/redo.

crates/ff-pdmodel/src/lib.rs

@@ -0,0 +1,14 @@
+//! ff-pdmodel — PD (Page Description) layer.
+//!
+//! Tầng tài liệu cao: Document/Page/Resources/ContentStream/TextRun/
+//! ImageObject/Annotation/AcroForm... Tương ứng "PD layer" của Adobe.
+//! Khung trống ở Phase 1; phát triển từ Phase 2 (annotation) trở đi.
+//!
+//! Xem docs/04-architecture.md.
+
+/// Kích thước trang theo điểm (points, 1/72 inch).
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct PageSize {
+    pub width: f32,
+    pub height: f32,
+}