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/CFeatureCheck.cmake

@@ -0,0 +1,39 @@
+# - Compile and run code to check for C features
+#
+# This functions compiles a source file under the `cmake` folder
+# and adds the corresponding `HAVE_[FILENAME]` flag to the CMake
+# environment
+#
+#  c_feature_check(<FLAG> [<VARIANT>])
+#
+# - Example
+#
+# include(CFeatureCheck)
+# c_feature_check(VLA)
+
+if(__c_feature_check)
+  return()
+endif()
+set(__c_feature_check INCLUDED)
+
+function(c_feature_check FILE)
+  string(TOLOWER ${FILE} FILE)
+  string(TOUPPER ${FILE} VAR)
+  string(TOUPPER "${VAR}_SUPPORTED" FEATURE)
+  if (DEFINED ${VAR}_SUPPORTED)
+    set(${VAR}_SUPPORTED 1 PARENT_SCOPE)
+    return()
+  endif()
+
+  if (NOT DEFINED COMPILE_${FEATURE})
+      message(STATUS "Performing Test ${FEATURE}")
+      try_compile(COMPILE_${FEATURE} ${PROJECT_BINARY_DIR} ${PROJECT_SOURCE_DIR}/cmake/${FILE}.c)
+  endif()
+
+  if(COMPILE_${FEATURE})
+    message(STATUS "Performing Test ${FEATURE} -- success")
+    set(${VAR}_SUPPORTED 1 PARENT_SCOPE)
+  else()
+    message(STATUS "Performing Test ${FEATURE} -- failed to compile")
+  endif()
+endfunction()