Add CI for Xcode 26.2

[swhitty]

Update GitHub Actions to build and test on Xcode 26.2 / macOS Tahoe by default.

Switch to official swift:6.2 image for linux

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.86%. Comparing base (02ad2f4) to head (c408fed).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
- Coverage   93.03%   90.86%   -2.17%     
==========================================
  Files          69       69              
  Lines        3448     3450       +2     
==========================================
- Hits         3208     3135      -73     
- Misses        240      315      +75     

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