[swhitty]

• Add linux_swift_6_4 (swift:6.4).
• Android and Windows now run on Swift 6.4 only: replace the skiptools 6.3 job with linux_swift_6_4_android (official 6.4.2 Android SDK, build only) and the 6.1.2 Windows job with windows_swift_6_4.
• Add xcode-27 (Xcode 27.2) as the default job that captures coverage; xcode_26_6 is now a plain build and test.

Mirrors the FlyingFox CI setup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.06%. Comparing base (c4190cf) to head (05eb449).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   89.04%   89.06%   +0.02%     
==========================================
  Files         167      167              
  Lines       13591    13589       -2     
==========================================
+ Hits        12102    12103       +1     
+ Misses       1489     1486       -3     

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