Fix duplicate availability macros in Tuist projects

[Kyle-Ye]

Summary

• Keep -index-ignore-system-modules unconditional for Tuist-generated projects.
• Document why applying a platform condition reintroduces duplicated Swift settings.

Why

Tuist emits both base and macOS SDK-specific OTHER_SWIFT_FLAGS when this setting is platform-conditioned. The SDK-specific entry inherits and repeats the shared flags, causing Swift 6.3 to receive each availability macro definition twice.

Validation

• Verified the generated Xcode project no longer contains an SDK-conditioned Swift flags entry.
• Verified availability macros appear once for both macOS and iOS targets.
• Verified the macOS UI test targets build and link with Xcode 26.6.

[Kyle-Ye]

/uitest update

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 15.35%. Comparing base (0dfa923) to head (72ed60b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1002   +/-   ##
=======================================
  Coverage   15.35%   15.35%           
=======================================
  Files         702      702           
  Lines       50398    50398           
=======================================
+ Hits         7737     7738    +1     
+ Misses      42661    42660    -1     

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