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/cmake/OpusConfig.cmake

@@ -0,0 +1,104 @@
+if(__opus_config)
+  return()
+endif()
+set(__opus_config INCLUDED)
+
+include(OpusFunctions)
+
+configure_file(cmake/config.h.cmake.in config.h @ONLY)
+add_definitions(-DHAVE_CONFIG_H)
+
+set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+set_property(GLOBAL PROPERTY C_STANDARD 99)
+
+if(MSVC)
+  add_definitions(-D_CRT_SECURE_NO_WARNINGS)
+endif()
+
+include(CheckLibraryExists)
+check_library_exists(m floor "" HAVE_LIBM)
+if(HAVE_LIBM)
+  list(APPEND OPUS_REQUIRED_LIBRARIES m)
+endif()
+
+include(CFeatureCheck)
+c_feature_check(VLA)
+
+include(CheckIncludeFile)
+check_include_file(alloca.h HAVE_ALLOCA_H)
+
+include(CheckSymbolExists)
+if(HAVE_ALLOCA_H)
+  add_definitions(-DHAVE_ALLOCA_H)
+  check_symbol_exists(alloca "alloca.h" USE_ALLOCA_SUPPORTED)
+else()
+  check_symbol_exists(alloca "stdlib.h;malloc.h" USE_ALLOCA_SUPPORTED)
+endif()
+
+include(CheckFunctionExists)
+check_function_exists(lrintf HAVE_LRINTF)
+check_function_exists(lrint HAVE_LRINT)
+
+if(CMAKE_SYSTEM_PROCESSOR MATCHES "(i[0-9]86|x86|X86|amd64|AMD64|x86_64)")
+  if(CMAKE_SIZEOF_VOID_P EQUAL 8)
+    set(OPUS_CPU_X64 1)
+  else()
+    set(OPUS_CPU_X86 1)
+  endif()
+elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "(arm|aarch64)")
+  set(OPUS_CPU_ARM 1)
+endif()
+
+if(NOT OPUS_DISABLE_INTRINSICS)
+  opus_supports_cpu_detection(RUNTIME_CPU_CAPABILITY_DETECTION)
+endif()
+
+if(OPUS_CPU_X86 OR OPUS_CPU_X64 AND NOT OPUS_DISABLE_INTRINSICS)
+  opus_detect_sse(COMPILER_SUPPORT_SIMD)
+elseif(OPUS_CPU_ARM AND NOT OPUS_DISABLE_INTRINSICS)
+  opus_detect_neon(COMPILER_SUPPORT_NEON)
+  if(COMPILER_SUPPORT_NEON)
+    option(OPUS_USE_NEON "Option to enable NEON" ON)
+    option(OPUS_MAY_HAVE_NEON "Does runtime check for neon support" ON)
+    option(OPUS_PRESUME_NEON "Assume target CPU has NEON support" OFF)
+    if(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
+      set(OPUS_PRESUME_NEON ON)
+    elseif(CMAKE_SYSTEM_NAME MATCHES "iOS")
+      set(OPUS_PRESUME_NEON ON)
+    endif()
+  endif()
+endif()
+
+if(MSVC)
+  check_flag(FAST_MATH /fp:fast)
+  check_flag(STACK_PROTECTOR /GS)
+  check_flag(STACK_PROTECTOR_DISABLED /GS-)
+else()
+  check_flag(FAST_MATH -ffast-math)
+  check_flag(STACK_PROTECTOR -fstack-protector-strong)
+  check_flag(HIDDEN_VISIBILITY -fvisibility=hidden)
+  set(FORTIFY_SOURCE_SUPPORTED 1)
+endif()
+
+if(MINGW)
+  # For MINGW we need to link ssp lib for security features such as
+  # stack protector and fortify_sources
+  check_library_exists(ssp __stack_chk_fail "" HAVE_LIBSSP)
+  if(NOT HAVE_LIBSSP)
+    message(WARNING "Could not find libssp in MinGW, disabling STACK_PROTECTOR and FORTIFY_SOURCE")
+    set(STACK_PROTECTOR_SUPPORTED 0)
+    set(FORTIFY_SOURCE_SUPPORTED 0)
+  endif()
+endif()
+
+if(NOT MSVC)
+  set(WARNING_LIST -Wall -W -Wstrict-prototypes -Wextra -Wcast-align -Wnested-externs -Wshadow)
+  include(CheckCCompilerFlag)
+  foreach(WARNING_FLAG ${WARNING_LIST})
+    string(REPLACE - "" WARNING_VAR ${WARNING_FLAG})
+    check_c_compiler_flag(${WARNING_FLAG} ${WARNING_VAR}_SUPPORTED)
+    if(${WARNING_VAR}_SUPPORTED)
+      add_compile_options(${WARNING_FLAG})
+    endif()
+  endforeach()
+endif()