fix(scripts): the xcframework never absorbs a Homebrew libopus

On a Mac with brew's opus installed, audiopus_sys found it via pkg-config and
statically linked it into the aarch64 slice — a lib built for the RUNNING
macOS (minos 26.0, tripping the script's own version guard) and existing only
for the host arch, so the x86_64 slice silently fell back to the vendored
build and the two slices shipped different libopus builds. Force the vendored
CMake build for every slice (OPUS_NO_PKG_CONFIG=1), with the CMake policy
floor modern CMake (>=4) needs to accept libopus's old cmake_minimum_required.

scripts/build-xcframework.sh

@@ -53,6 +53,15 @@ if [[ -z "${DEVELOPER_DIR:-}" ]]; then
     esac # a non-beta xcode-select default is fine as-is
 fi
 
+# Hermetic Opus: never let audiopus_sys link a Homebrew libopus via pkg-config. A brew lib
+# is built for the RUNNING macOS (its objects carry that minos, tripping the version guard
+# below) and only exists for the host arch — the other slice silently falls back to the
+# vendored build, so the two slices ship different libopus builds. Force the vendored CMake
+# build everywhere; the policy floor keeps modern CMake (≥4) accepting libopus's old
+# `cmake_minimum_required`.
+export OPUS_NO_PKG_CONFIG=1
+export CMAKE_POLICY_VERSION_MINIMUM=3.5
+
 # Deployment targets must match Package.swift's platforms, or every consumer link emits
 # "object file was built for newer macOS version" warnings.
 for t in "${TARGETS_MAC[@]}"; do