Add unmapped_glyph_names configuration field to DifferencesOverlay struct,
which serves as the CMAP generator for Type1 font encoding differences.
Changes:
- Added unmapped_glyph_names: HashSet<String> field to DifferencesOverlay
- Added with_unmapped_glyph_names() constructor for custom configuration
- Added is_unmapped_glyph_name() instance method for checking membership
- Added unmapped_glyph_names() getter and set_unmapped_glyph_names() setter
- Updated parse() to use instance method instead of global function
- Defaults to global UNMAPPED_GLYPH_NAMES set when not explicitly configured
Acceptance criteria:
- CMAP generator struct has unmapped_glyph_names field
- Field is HashSet<String> for O(1) lookup efficiency
- Field can be set via constructor or setter method
- Code compiles and all tests pass (24 encoding tests)
Closes bf-4zgpr. See notes/bf-4zgpr.md for verification details.
