Update CI toolchain versions

[Kyle-Ye]

Summary

• Update CI and package toolchain pins to Xcode 26.3 / Swift 6.2.4.
• Keep macOS jobs on the macos-15 runner and iOS simulator jobs on iOS 18.5.
• Update related Swift checkout/configuration references for the new toolchain.
• Add Example setup.sh

Dependency

OpenSwiftUIProject/DarwinPrivateFrameworks#66

[augmentcode]

🤖 Augment PR Summary

Summary: Updates the repository’s CI and package/tooling pins to the newer Swift/Xcode toolchain.

Changes:

• Bump GitHub Actions workflows to use Xcode 26.3 on macOS jobs and Swift 6.2.4 for Linux container jobs.
• Update the Swift headers submodule to the 6.2 release branch/commit.
• Advance swift-tools-version declarations (Package.swift/Tuist) and SwiftFormat’s configured Swift version to 6.2.
• Refresh README build guidance to match the new recommended toolchain.

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[augmentcode]

Review completed. 2 suggestions posted.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.17%. Comparing base (6166503) to head (99deb4e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
+ Coverage   32.10%   32.17%   +0.07%     
==========================================
  Files          71       71              
  Lines        2601     2601              
==========================================
+ Hits          835      837       +2     
+ Misses       1766     1764       -2     

Flag	Coverage Δ
ios	32.19% <ø> (+0.07%)	⬆️
macos	10.15% <ø> (ø)
ubuntu	12.03% <ø> (ø)

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see 1 file with indirect coverage changes

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