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/.appveyor.yml

@@ -0,0 +1,37 @@
+image: Visual Studio 2015
+configuration:
+- Debug
+- DebugDLL
+- DebugDLL_fixed
+- Release
+- ReleaseDLL
+- ReleaseDLL_fixed
+
+platform:
+- Win32
+- x64
+
+environment:
+  api_key:
+    secure: kR3Ac0NjGwFnTmXdFrR8d6VXjdk5F7L4F/BilC4nvaM=
+
+build:
+  project: win32\VS2015\opus.sln
+  parallel: true
+  verbosity: minimal
+
+after_build:
+- cd %APPVEYOR_BUILD_FOLDER%
+- 7z a opus.zip win32\VS2015\%PLATFORM%\%CONFIGURATION%\opus.??? include\*.h
+
+test_script:
+- cd %APPVEYOR_BUILD_FOLDER%\win32\VS2015\%PLATFORM%\%CONFIGURATION%
+- test_opus_api.exe
+- test_opus_decode.exe
+- test_opus_encode.exe
+
+artifacts:
+- path: opus.zip
+
+on_success:
+- ps: if ($env:api_key -and "$env:configuration/$env:platform" -eq "ReleaseDLL_fixed/x64") { Start-AppveyorBuild -ApiKey $env:api_key -ProjectSlug 'opus-tools' }