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-cos/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "ff-cos"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "FoFreeXit COS layer: low-level PDF object model (dict/array/stream/xref)"
+
+[dependencies]
+thiserror.workspace = true