destinationFilters are silently ignored for syncedFolder sources (no platformFiltersByRelativePath emitted)

[konstantin-mapbox]

Summary

destinationFilters on a TargetSource work for regular (group) sources: each file gets platformFilters on its PBXBuildFile. For type: syncedFolder the same key is silently ignored, so every file in the folder compiles for all destinations of the target.

Xcode itself supports per-file platform filters inside synced folders via platformFiltersByRelativePath on PBXFileSystemSynchronizedBuildFileExceptionSet, and the bundled XcodeProj already models that field (tuist/XcodeProj#1065, in XcodeProj 9.8.0+).

Reproduction

name: Demo
targets:
  App:
    type: application
    supportedDestinations: [iOS, visionOS]
    sources:
      - path: Sources
        type: syncedFolder
        excludes:
          - VisionOSMain.swift
        destinationFilters: [iOS]
      - path: Sources
        type: syncedFolder
        includes:
          - VisionOSMain.swift
        destinationFilters: [visionOS]

Sources/AppDelegate.swift uses UIScreen, Sources/VisionOSMain.swift is a visionOS @main.

Expected

The generated exception set carries the filters, the way Xcode writes them when you uncheck "Any Supported Platform" in the file inspector:

platformFiltersByRelativePath = {
    AppDelegate.swift = (ios, );
    VisionOSMain.swift = (xros, );
};

and the visionOS build does not compile AppDelegate.swift.

Actual

membershipExceptions are emitted from excludes/includes, but there is no platformFiltersByRelativePath at all.
Building for visionOS compiles AppDelegate.swift and fails with 'UIScreen' is unavailable in visionOS. No warning that the key is ignored.

I verified that Xcode honors a hand-written platformFiltersByRelativePath in the generated project (Xcode 26.5): the filtered files are skipped on the other platform for both iOS and visionOS builds.
Folder keys are not honored by Xcode, only file paths, so the generator would need to enumerate matching files the same way syncedFolderExceptions already does for excludes.

Environment

• XcodeGen 2.45.4 (same on 2.46.0 / master)
• Xcode 26.5
• macOS 26

Related, but different: #1589 / #1587 (excludes), #1602 / #1604 (includes), #1607 (directory-level exceptions).