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.
• Remove the obsolete Linux toolchain patch script and workflow calls.

Dependency

OpenSwiftUIProject/DarwinPrivateFrameworks#66

Validation

• Rebases cleanly on origin/main.
• Ran git diff --check origin/main..HEAD.
• Confirmed no fix-toolchain references remain.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (7a963a0) to head (cf39609).

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #27   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          8       8           
  Lines        356     356           
=====================================
  Misses       356     356           

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