Fix iOS workflow asset preparation

[Copilot]

The iOS Actions job failed during host-side generation because default.xex was not available at the CMake-expected path.

• Asset discovery

  • Locate assets recursively when the private repository uses a nested directory.
  • Copy the discovered asset set into UnleashedRecompLib/private.

• Pre-build validation

  • Require both default.xex and default.xexp before starting CMake generation.

asset_file="$(find ./private -name default.xex -type f -print -quit)"
asset_dir="$(dirname "${asset_file}")"
cp -RL "${asset_dir}"/* ./UnleashedRecompLib/private/