Update README build badge

[Code0x58]

Summary

• add a GitHub Actions build badge to the README in place of the legacy Travis reference

Testing

• not run (not requested)

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.83%. Comparing base (e487984) to head (1d65bc8).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #11   +/-   ##
=======================================
  Coverage   95.83%   95.83%           
=======================================
  Files           1        1           
  Lines         144      144           
=======================================
  Hits          138      138           
  Misses          6        6           

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