Add assertions to test_cmap_unmapped_glyph_skip to verify that unmapped
glyphs (configured in build/unmapped-glyph-names.json as g001-g003,
.notdef, .null) are absent from the CMAP output structure.
Changes:
- Add count verification: Ensures CMAP contains exactly the expected
number of mappings, detecting spurious entries
- Add validity check: Iterates over all entries verifying valid Unicode
destinations, with clear failure messages
Assertion messages identify which mapping should be absent via byte
sequence inspection, meeting the bead's acceptance criteria.
Verification: notes/bf-5d2id.md
Closes: bf-5d2id
