Add support for cmap format 12 (Unicode beyond BMP)

[manh-dan]

Summary

The library currently does not support cmap format 12, which is required to map Unicode codepoints beyond U+FFFF (e.g., CJK Extension B characters).

Problem

Fonts containing characters outside the Basic Multilingual Plane (BMP), such as 𠮷 (U+20BB7), cannot be correctly mapped to glyphs.

Root Cause

Only cmap format 4 (16-bit) is supported, which limits Unicode support to BMP.

Proposed Solution

• Implement parsing for cmap format 12
• Support 32-bit Unicode codepoints
• Return glyph mappings for codepoints beyond U+FFFF