fix: prevent panic when embedding fonts

[pamburus]

Replace u64 bitmap with BitVec to support 256+ fonts without overflow when using --embed-fonts option.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.21%. Comparing base (082a3e5) to head (766bed1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #531      +/-   ##
==========================================
+ Coverage   80.18%   80.21%   +0.02%     
==========================================
  Files          31       31              
  Lines        4159     4164       +5     
==========================================
+ Hits         3335     3340       +5     
  Misses        824      824              

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