Add SHA256 checksums to releases

[vertti]

Summary

• Generate checksums.txt with SHA256 hashes in release workflow
• Update install.sh to verify checksum before installing

Install script changes

The installer now:

1. Downloads checksums.txt from the release
2. Downloads the binary
3. Verifies the SHA256 checksum (uses sha256sum or shasum depending on platform)
4. Only installs if verification passes

Manual verification

Users can also verify downloads manually:

sha256sum -c checksums.txt --ignore-missing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.72%. Comparing base (72407ff) to head (4e2ccc0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   88.72%   88.72%           
=======================================
  Files          31       31           
  Lines        1313     1313           
=======================================
  Hits         1165     1165           
  Misses        108      108           
  Partials       40       40           

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