fix: correct arithmetic coder interval normalisation

[danieleades]

Summary

• initialise the arithmetic coding state with an inclusive upper bound
• restore the implicit top bit during encoder and decoder renormalisation steps

Testing

• cargo test

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https://chatgpt.com/codex/tasks/task_e_68f3f919b6d0832abd98e68b1e637c31

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.57%. Comparing base (6ef8a5b) to head (ac034f8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #106   +/-   ##
=======================================
  Coverage   87.57%   87.57%           
=======================================
  Files          11       11           
  Lines         483      483           
=======================================
  Hits          423      423           
  Misses         60       60           

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