docs: enable emoji rendering in latex builds

[i-oden]

Summary

• add sphinxemoji extension with twemoji style
• configure LaTeX build to use xelatex and emoji package
• include sphinxemoji in dev requirements

Testing

• pytest (fails: ModuleNotFoundError: No module named 'pyfakefs')
• make -C docs latexpdf (fails: command not found: sphinx-build)

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https://chatgpt.com/codex/tasks/task_b_68bebce9df9483269a94f2a2cb6d4f40

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.91%. Comparing base (da1af8a) to head (f251874).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #841   +/-   ##
=======================================
  Coverage   56.91%   56.91%           
=======================================
  Files          63       63           
  Lines        3681     3681           
=======================================
  Hits         2095     2095           
  Misses       1586     1586           

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