Lint with Swift 6.4's swift-format

[germ-mark]

The lint job used the macos-15 runner's Xcode toolchain (Swift 6.2.x), whose swift-format orders an @_spi(…) @testable import line differently from the 6.4 formatter the code is written with. #109 passes lint locally under 6.3 and 6.4 but fails in CI on OrderedImports.

The lint job now runs in the swift:6.4.0 container; swift format ships in the toolchain. The build and test jobs are unchanged, so they still prove the 6.2.3 minimum. All of main already lints clean under 6.4.

CI-only; no changeset.

🤖 Generated with Claude Code

[changeset-bot]

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