[CI] Enable Swift format check

```
** Running swift format lint...
Sources/ComplexModule/Complex+AlgebraicField.swift:61:11: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'wʹ': ʹ
Sources/ComplexModule/Complex+AlgebraicField.swift:62:11: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'zʹ': ʹ
Sources/ComplexModule/Complex+AlgebraicField.swift:114:9: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'wʹ': ʹ
Sources/ComplexModule/Complex+AlgebraicField.swift:115:9: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'zʹ': ʹ
Sources/ComplexModule/Complex+ElementaryFunctions.swift:238:9: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'θ': θ
Sources/ComplexModule/Complex+ElementaryFunctions.swift:341:9: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'θ': θ
Sources/ComplexModule/Complex+ElementaryFunctions.swift:358:9: warning: [IdentifiersMustBeASCII] remove non-ASCII characters from 'y²': ²
Sources/_TestSupport/DoubleWidth.swift:305:9: warning: [AlwaysUseLowerCamelCase] rename the constant 'quotient_' using lowerCamelCase
Sources/_TestSupport/DoubleWidth.swift:311:9: warning: [AlwaysUseLowerCamelCase] rename the constant 'remainder_' using lowerCamelCase
Sources/_TestSupport/DoubleWidth.swift:395:9: warning: [AlwaysUseLowerCamelCase] rename the constant 'quotient_' using lowerCamelCase
Sources/_TestSupport/DoubleWidth.swift:401:9: warning: [AlwaysUseLowerCamelCase] rename the constant 'remainder_' using lowerCamelCase
Sources/_TestSupport/DoubleWidth.swift:666:11: warning: [AlwaysUseLowerCamelCase] rename the constant 'lhs_' using lowerCamelCase
Tests/IntegerUtilitiesTests/DoubleWidthTests.swift:420:8: warning: [AlwaysUseLowerCamelCase] rename the function 'testConversions_ToAndFromString' using lowerCamelCase
```

Start by reviewing the Swift format warnings in Sources/ComplexModule/Complex+AlgebraicField.swift, Sources/ComplexModule/Complex+ElementaryFunctions.swift, Sources/_TestSupport/DoubleWidth.swift, and Tests/IntegerUtilitiesTests/DoubleWidthTests.swift. Run the existing Swift format lint command and address the listed identifier warnings, then enable the format check in the repository’s CI configuration so the check completes without warnings.

Written by the indexing model from the issue text.