fix(windows): vendor audiopus_sys + patch libopus for clang-cl SIMD

cargo-xwin drives the Windows MSVC cross-compile via clang-cl, under
which CMake sets MSVC=1 — causing libopus 1.3.1's `if(NOT MSVC)` guards
to skip the per-file `-msse4.1` / `-mssse3` COMPILE_FLAGS that its x86
SIMD source files need. Clang-cl (unlike real cl.exe) still honors
Clang's target-feature system, so those files then fail to compile
with "always_inline function '_mm_cvtepi16_epi32' requires target
feature 'sse4.1'" errors across silk/NSQ_sse4_1.c, NSQ_del_dec_sse4_1.c,
and VQ_WMat_EC_sse4_1.c.

Earlier attempts to fix this downstream (cargo-xwin toolchain file,
override.cmake CMAKE_C_COMPILE_OBJECT <FLAGS> replace, CFLAGS env vars)
all failed because cargo-xwin rewrites override.cmake from scratch on
every `cargo xwin build` invocation and cmake-rs's -DCMAKE_C_FLAGS=
assembly happens before toolchain FORCE sets propagate.

Fixing it upstream at the source: vendor audiopus_sys 0.2.2 into
vendor/audiopus_sys, patch its bundled opus/CMakeLists.txt to introduce
an MSVC_CL var (true only when CMAKE_C_COMPILER_ID == "MSVC", i.e. real
cl.exe), and flip the eight `if(NOT MSVC)` SIMD guards to
`if(NOT MSVC_CL)`. Clang-cl then gets the GCC-style per-file flags and
the SSE4.1 sources build cleanly. Also flip the `if(MSVC)` global /arch
block at line 445 to `if(MSVC_CL)` so only cl.exe applies /arch:AVX and
clang-cl relies purely on per-file flags (no global/per-file mixing).

Wire via [patch.crates-io] in the workspace root Cargo.toml; the patch
is resolved relative to the workspace root as `vendor/audiopus_sys`.

Upstream context: xiph/opus#256, xiph/opus PR #257 (both stale).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/audiopus_sys/opus/.gitlab-ci.yml

@@ -0,0 +1,61 @@
+include:
+  - template: 'Workflows/Branch-Pipelines.gitlab-ci.yml'
+
+default:
+  tags:
+    - docker
+  # Image from https://hub.docker.com/_/gcc/ based on Debian
+  image: gcc:9
+
+whitespace:
+  stage: test
+  script:
+    - git diff-tree --check origin/master HEAD
+
+autoconf:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y zip doxygen
+  script:
+    - ./autogen.sh
+    - ./configure
+    - make -j4
+    - make distcheck
+  cache:
+    paths:
+      - "src/*.o"
+      - "src/.libs/*.o"
+      - "silk/*.o"
+      - "silk/.libs/*.o"
+      - "celt/*.o"
+      - "celt/.libs/*.o"
+
+cmake:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y cmake ninja-build
+  script:
+    - mkdir build
+    - cmake -S . -B build -G "Ninja" -DCMAKE_BUILD_TYPE=Release -DOPUS_BUILD_TESTING=ON -DOPUS_BUILD_PROGRAMS=ON
+    - cmake --build build
+    - cd build && ctest --output-on-failure
+
+meson:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y python3-pip ninja-build doxygen
+    - export XDG_CACHE_HOME=$PWD/pip-cache
+    - pip3 install --user meson
+  script:
+    - export PATH=$PATH:$HOME/.local/bin
+    - mkdir builddir
+    - meson setup --werror -Dtests=enabled -Ddocs=enabled -Dbuildtype=release builddir
+    - meson compile -C builddir
+    - meson test -C builddir
+    #- meson dist --no-tests -C builddir
+  cache:
+    paths:
+      - 'pip-cache/*'