Update toolchains for Xcode 26.6 and Swift 6.3.3

[Kyle-Ye]

Summary

• Move package manifests and CI to Swift 6.3 / Swift 6.3.3 and Xcode 26.6.
• Remove Swift 6.2 index-store workarounds from SwiftPM, Xcode, and Tuist paths.
• Update the recommended toolchain documentation.

Dependency

• Depends on OpenSwiftUIProject/DarwinPrivateFrameworks#69 for the Swift 6.3.3-compatible generated interfaces used by the builds.

Impact

This keeps package metadata and CI aligned with the Swift 6.3.3 / Xcode 26.6 toolchain.

Validation

Changed package and workflow manifests parse successfully with Swift 6.3.3 and Xcode 26.6.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.10%. Comparing base (49a4009) to head (b9dce47).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
- Coverage   32.17%   32.10%   -0.08%     
==========================================
  Files          71       71              
  Lines        2601     2601              
==========================================
- Hits          837      835       -2     
- Misses       1764     1766       +2     

Flag	Coverage Δ
ios	32.11% <ø> (-0.08%)	⬇️
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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[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Cache hit rate	Tests	Skipped	Ran	Commit
OpenAttributeGraph-Package	✅	0 %	137	0	137	02e7f7b18

Builds 🔨

Scheme	Status	Duration	Commit
OpenAttributeGraph	✅	24.1s	02e7f7b18
OpenAttributeGraph-Package	✅	40.7s	02e7f7b18