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/OpusPackageVersion.cmake

@@ -0,0 +1,70 @@
+if(__opus_version)
+  return()
+endif()
+set(__opus_version INCLUDED)
+
+function(get_package_version PACKAGE_VERSION PROJECT_VERSION)
+
+  find_package(Git)
+  if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_LIST_DIR}/.git")
+    execute_process(COMMAND ${GIT_EXECUTABLE}
+                    --git-dir=${CMAKE_CURRENT_LIST_DIR}/.git describe
+                    --tags --match "v*" OUTPUT_VARIABLE OPUS_PACKAGE_VERSION)
+    if(OPUS_PACKAGE_VERSION)
+      string(STRIP ${OPUS_PACKAGE_VERSION}, OPUS_PACKAGE_VERSION)
+      string(REPLACE \n
+                     ""
+                     OPUS_PACKAGE_VERSION
+                     ${OPUS_PACKAGE_VERSION})
+      string(REPLACE ,
+                     ""
+                     OPUS_PACKAGE_VERSION
+                     ${OPUS_PACKAGE_VERSION})
+
+      string(SUBSTRING ${OPUS_PACKAGE_VERSION}
+                       1
+                       -1
+                       OPUS_PACKAGE_VERSION)
+      message(STATUS "Opus package version from git repo: ${OPUS_PACKAGE_VERSION}")
+    endif()
+
+  elseif(EXISTS "${CMAKE_CURRENT_LIST_DIR}/package_version"
+         AND NOT OPUS_PACKAGE_VERSION)
+    # Not a git repo, lets' try to parse it from package_version file if exists
+    file(STRINGS package_version OPUS_PACKAGE_VERSION
+         LIMIT_COUNT 1
+         REGEX "PACKAGE_VERSION=")
+    string(REPLACE "PACKAGE_VERSION="
+                   ""
+                   OPUS_PACKAGE_VERSION
+                   ${OPUS_PACKAGE_VERSION})
+    string(REPLACE "\""
+                   ""
+                   OPUS_PACKAGE_VERSION
+                   ${OPUS_PACKAGE_VERSION})
+    # In case we have a unknown dist here we just replace it with 0
+    string(REPLACE "unknown"
+                   "0"
+                   OPUS_PACKAGE_VERSION
+                   ${OPUS_PACKAGE_VERSION})
+      message(STATUS "Opus package version from package_version file: ${OPUS_PACKAGE_VERSION}")
+  endif()
+
+  if(OPUS_PACKAGE_VERSION)
+    string(REGEX
+      REPLACE "^([0-9]+.[0-9]+\\.?([0-9]+)?).*"
+               "\\1"
+               OPUS_PROJECT_VERSION
+               ${OPUS_PACKAGE_VERSION})
+  else()
+    # fail to parse version from git and package version
+    message(WARNING "Could not get package version.")
+    set(OPUS_PACKAGE_VERSION 0)
+    set(OPUS_PROJECT_VERSION 0)
+  endif()
+
+  message(STATUS "Opus project version: ${OPUS_PROJECT_VERSION}")
+
+  set(PACKAGE_VERSION ${OPUS_PACKAGE_VERSION} PARENT_SCOPE)
+  set(PROJECT_VERSION ${OPUS_PROJECT_VERSION} PARENT_SCOPE)
+endfunction()