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/doc/meson.build

@@ -0,0 +1,33 @@
+have_dot = find_program('dot', required: false).found()
+
+doxyfile_conf = configuration_data()
+doxyfile_conf.set('VERSION', opus_version)
+doxyfile_conf.set('HAVE_DOT', have_dot)
+doxyfile_conf.set('top_srcdir', top_srcdir)
+doxyfile_conf.set('top_builddir', top_builddir)
+
+doxyfile = configure_file(input: 'Doxyfile.in',
+  output: 'Doxyfile',
+  configuration: doxyfile_conf,
+  install: false)
+
+docdir = join_paths(get_option('datadir'), get_option('docdir'))
+
+doc_inputs = [
+  'customdoxygen.css',
+  'footer.html',
+  'header.html',
+  'opus_logo.svg',
+  top_srcdir + '/include/opus.h',
+  top_srcdir + '/include/opus_multistream.h',
+  top_srcdir + '/include/opus_defines.h',
+  top_srcdir + '/include/opus_types.h',
+  top_srcdir + '/include/opus_custom.h',
+]
+
+custom_target('doc',
+  input: [ doxyfile ] + doc_inputs,
+  output: [ 'html' ],
+  command: [ doxygen, doxyfile ],
+  install_dir: docdir,
+  install: true)