Add comprehensive documentation of all CMAP and ToUnicode entry creation
locations in the pdftract codebase. Identifies key insertion points for
future skip logic for unmapped glyphs.
Key locations documented:
- CMAP codespace range creation (cmap/codespace.rs:60, 356)
- ToUnicode mapping creation (font/cmap.rs:86)
- Type1 font encoding differences (font/encoding.rs:197)
- ToUnicode resolution success (font/resolver.rs:398)
All locations already have MARKER comments in code referencing this note.
Closes bf-e4uvb
