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 and package-prebuilt workarounds.
• Add SDK 26.4 and 26.5 build-version constants and refresh linked-version expectations.
• Update the recommended project toolchain documentation.

Dependency

• DarwinPrivateFrameworks#69
• OpenAttributeGraph#234

Impact

This aligns package metadata, CI, and SDK-linked behavior checks with the Swift 6.3.3 / Xcode 26.6 toolchain.

Validation

Changed package and workflow manifests, JSON configuration, and shell scripts parse successfully. SDK-linked expectations were checked against the Xcode 26.6 XCTest binaries.

[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Cache hit rate	Tests	Skipped	Ran	Commit
OpenSwiftUI	✅	0 %	1014	0	1014	b8dd677cb

Builds 🔨

Scheme	Status	Duration	Commit
OpenSwiftUI	✅	1m 26s	b8dd677cb

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.24%. Comparing base (76597a1) to head (ee17732).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #987      +/-   ##
==========================================
+ Coverage   26.23%   26.24%   +0.01%     
==========================================
  Files         734      734              
  Lines       55580    55580              
==========================================
+ Hits        14579    14588       +9     
+ Misses      41001    40992       -9     

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