Fix macOS superbuild for Apple Silicon toolchains

[chraibi]

Three failures prevented cmake --build _superbuild from completing on macOS (Apple Silicon, Xcode 26 / AppleClang 21). All changes are inside the existing elseif(APPLE) branches of UnrealFolder/ProjectMobius/CMakeLists.txt — Windows and Linux paths are untouched.

1. Assimp -Werror vs a newer compiler

SceneCombiner.cpp:997:14: error: first argument in call to 'memcpy' is a pointer to
  non-trivially copyable type 'aiFace' [-Werror,-Wnontrivial-memcall]

Assimp 5.4.3 predates AppleClang's -Wnontrivial-memcall, and its own ASSIMP_WARNINGS_AS_ERRORS (default ON) turns the new diagnostic into a build failure. Vendored third-party source is left untouched; the option is disabled from the superbuild instead.

2. minizip architecture mismatch

ld: warning: ignoring file '/usr/local/Cellar/minizip/1.3.1/lib/libminizip.dylib':
  found architecture 'x86_64', required architecture 'arm64'
Undefined symbols for architecture arm64: _unzClose, _unzGetCurrentFileInfo, ...

Assimp runs use_pkgconfig(UNZIP minizip), which on a machine with an Intel Homebrew prefix at /usr/local resolves to an x86_64 library — unlinkable against the arm64 build this branch pins. ASSIMP_BUILD_MINIZIP=ON skips the lookup and compiles the bundled contrib/unzip.

3. Staging produced a nested directory

if(EXISTS "${ASSIMP_INST}/lib/Mac") is evaluated at configure time, before Assimp has been built or installed. On a clean tree the directory does not exist yet, so it fell back to lib/ and copy_directory staged Plugins/UE4_Assimp/Binaries/Mac/Mac/ — one level too deep. The same branch already forces CMAKE_INSTALL_LIBDIR=lib/Mac, so the layout is known and the check was removable.

This one is latent rather than fatal: UE_AssimpLibrary.Build.cs reads from assimp/lib/Mac, not Binaries/Mac, so the editor build succeeds either way. It would surface at packaging time.

Verification

• Full superbuild completes, exit 0. Assimp and HDF5 both produced as arm64 (lipo -archs), staged flat into Plugins/UE4_Assimp/Binaries/Mac/.
• ProjectMobiusEditor Mac Development builds against UE 5.5.4 — 191/191 actions, exit 0 — and links the resulting Assimp dylib.
• The editor then launches and loads an HDF5 simulation successfully.

Not tested on Windows or Linux; those code paths are unmodified.

Note for macOS builders

Two further things are needed on a current macOS, deliberately not included here since they are environment rather than repo concerns:

• UE 5.5 caps Xcode at 16.9.0 in Engine/Config/Apple/Apple_SDK.json. With Xcode 26 installed, UBT reports Platform Mac is not a valid platform to build.
• Xcode 26 ships the Metal shader compiler as a separate component: xcodebuild -downloadComponent MetalToolchain.

[Copilot]

Pull request overview

This PR updates the macOS (Apple Silicon) superbuild configuration to ensure cmake --build _superbuild completes successfully with newer Apple toolchains, by adjusting how Assimp is configured and staged in the existing elseif(APPLE) branches.

Changes:

• Disable Assimp’s ASSIMP_WARNINGS_AS_ERRORS on macOS to avoid new AppleClang warnings becoming hard failures.
• Force Assimp to build its bundled unzip/minizip path (ASSIMP_BUILD_MINIZIP=ON) to avoid linking against an x86_64 Homebrew minizip.
• Remove the configure-time EXISTS check for Assimp’s lib layout and stage from the known lib/Mac install directory to prevent nested Mac/Mac staging.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[chraibi]

Additional verification on a pristine clone (the case the staging bug actually hits):

$ git clone <fork> && cd UnrealFolder/ProjectMobius
$ cmake -S . -B _superbuild -G Ninja -DCMAKE_BUILD_TYPE=Release && cmake --build _superbuild --parallel
exit=0, no errors

$ lipo -archs Plugins/UE4_Assimp/Binaries/Mac/libassimp.5.4.3.dylib   -> arm64
$ lipo -archs Plugins/Hdf5DataPlugin/.../install/lib/libhdf5.a        -> arm64
$ ls Plugins/UE4_Assimp/Binaries/Mac/
cmake  libassimp.5.4.3.dylib  libassimp.5.dylib  libassimp.dylib  pkgconfig

Staging is flat. On a fresh checkout there is no pre-existing lib/Mac for the configure-time EXISTS check to find, so the old code took the else branch and produced Binaries/Mac/Mac/ — this run confirms the third fix on exactly that path, rather than on a tree that happened to have been built before.

[chraibi]

Superseded by #22: the Assimp fixes here are re-applied there against the rewritten superbuild on the dev branch, the staging fix is already absorbed into that branch, and #22 additionally forces arm64 for the rewritten HDF5 step. Closing to keep the queue clean ahead of the dev→main merge.