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/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "ff-pdmodel"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "FoFreeXit PD layer: high-level document model (page/text/image/annotation)"
+
+[dependencies]
+ff-cos = { path = "../ff-cos" }
+thiserror.workspace = true