Verify generated code compiles across Swift language modes

[rozd]

Summary

• Add compilation verification that generated code builds under all 4 combinations of Swift language mode (5/6) and default actor isolation (nonisolated/MainActor)
• Replace committed generated fixture files with a build tool plugin that generates them at build time from theme.json
• Add --skip-defaults flag to ThemeKitGeneratorCLI so the plugin can skip the placeholder defaults file

Test plan

• swift build succeeds — all 4 fixture targets compile with plugin-generated code
• swift test passes — all 219 tests pass including GeneratedCodeCompilationTests
• Verify plugin is internal (not exported as a product) and invisible to consumers

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.91%. Comparing base (e2148fe) to head (d9057eb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   98.89%   98.91%   +0.01%     
==========================================
  Files          35       36       +1     
  Lines        2178     2202      +24     
==========================================
+ Hits         2154     2178      +24     
  Misses         24       24              

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