Swift Package Manager Support with IGListKit Integration

[3a4oT]

Summary

This PR adds Swift Package Manager support for Texture with IGListKit integration.

Changes

Package.swift:

• Add SPM manifest with source distribution
• Support for AsyncDisplayKit and TextureIGListKitExtensions products
• IGListKit integration disabled at compile-time (AS_IG_LIST_KIT=0) for source builds
• Dependencies: PINRemoteImage 3.0.4, IGListKit 5.0.0
• Traits approach was explored but removed due to limitations with Objective-C/C++ mixed packages

TextureIGListKitExtensions module:

• Pure Swift wrapper for IGListKit integration
• Provides adapter.setCollectionNode(node) as Swift-friendly alternative to Objective-C setASDKCollectionNode:
• Runtime conformance to ASCollectionDataSourceInterop/ASCollectionDelegateInterop protocols
• Updated for IGListKit 5.0+ (uses allowsBackgroundDiffing instead of removed allowsBackgroundReloading)

Build infrastructure:

• SPM source layout generator script with automatic module.modulemap generation
• Eliminates umbrella header warnings for private headers
• Build script modes: spm-texture-basic, spm-texture-iglistkit, spm-app-iglistkit
• CI workflow updates for Xcode 16.4

Examples:

• SPMBasic - Basic SPM integration test
• SPMWithIGListKit - Library package with IGListKit
• ASIGListKitSPM - iOS app example

Why Swift wrapper is needed

SPM cannot export Objective-C categories on classes from external modules. The original IGListAdapter+AsyncDisplayKit category extends IGListAdapter from the IGListKit module, which SPM source builds cannot compile. The Swift implementation provides the same functionality while working within SPM's constraints.

Design decisions

Traits approach: Initially explored using SPM traits for optional IGListKit support, but this was removed because:

• Traits don't work with Objective-C conditional compilation (#if directives)
• Mixed Objective-C/C++ codebases have module map limitations with traits
• Simpler user experience: just import TextureIGListKitExtensions when needed

IGListKit always available: IGListKit dependencies are included but compile-time flags are disabled (AS_IG_LIST_KIT=0), allowing users to opt-in via the Swift wrapper module.