Fix TextBuilder stack overflow with many attributed runs

[markawilliamson]

Summary

• Concatenate Text fragments with partialResult + text instead of localized interpolation
• Add a macOS regression test that renders 2,500 attributed runs and forces SwiftUI text resolution

This avoids building a recursively nested LocalizedStringKey/Text interpolation tree when TextBuilder combines many attributed runs.

Fixes #64.

Testing

• swift test --filter TextBuilderTests
• swift test --skip CodeTokenizerTests

I also ran unfiltered swift test; it built successfully but my local run failed in the existing CodeTokenizerTests because the tokenizer was nil, unrelated to this change.

[gonzalezreal]

Hi @markawilliamson,

Thanks for the fix and the regression test! ❤️
CI failures are unrelated to this PR and already fixed in main.