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/win32/genversion.bat

@@ -0,0 +1,37 @@
+@echo off
+
+setlocal enableextensions enabledelayedexpansion
+
+for /f %%v in ('cd "%~dp0.." ^&^& git status ^>NUL 2^>NUL ^&^& git describe --tags --match "v*" --dirty 2^>NUL') do set version=%%v
+
+if not "%version%"=="" set version=!version:~1! && goto :gotversion
+
+if exist "%~dp0..\package_version" goto :getversion
+
+echo Git cannot be found, nor can package_version. Generating unknown version.
+
+set version=unknown
+
+goto :gotversion
+
+:getversion
+
+for /f "delims== tokens=2" %%v in (%~dps0..\package_version) do set version=%%v
+set version=!version:"=!
+
+:gotversion
+
+set version=!version: =!
+set version_out=#define %~2 "%version%"
+
+echo %version_out%> "%~1_temp"
+
+echo n | comp "%~1_temp" "%~1" > NUL 2> NUL
+
+if not errorlevel 1 goto exit
+
+copy /y "%~1_temp" "%~1"
+
+:exit
+
+del "%~1_temp"