Located and documented the exact locations where ToUnicode entries are created:
- Primary storage: ToUnicodeMap::add_mapping() in cmap.rs:86-88
- Resolution entry: resolve_level1() in resolver.rs:397-401
- Documented data flow from PDF dictionaries through text extraction
- Identified existing filtering logic (empty/U+FFFD entries)
- Identified where to add future skip logic for unmapped glyphs
Closes bf-1pq0q. Verification: notes/bf-1pq0q.md.
