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/tests/meson.build

@@ -0,0 +1,34 @@
+# Tests that link to libopus
+opus_tests = [
+  ['test_opus_api'],
+  ['test_opus_decode', [], 60],
+  ['test_opus_encode', 'opus_encode_regressions.c', 120],
+  ['test_opus_padding'],
+  ['test_opus_projection'],
+]
+
+foreach t : opus_tests
+  test_name = t.get(0)
+  extra_srcs = t.get(1, [])
+
+  test_kwargs = {}
+  if t.length() > 2
+    test_kwargs += {'timeout': t[2]}
+  endif
+
+  exe_kwargs = {}
+  # This test uses private symbols
+  if test_name == 'test_opus_projection'
+    exe_kwargs = {
+      'link_with': [celt_lib, silk_lib],
+      'objects': opus_lib.extract_all_objects(),
+    }
+  endif
+
+  exe = executable(test_name, '@0@.c'.format(test_name), extra_srcs,
+    include_directories: opus_includes,
+    dependencies: [libm, opus_dep],
+    install: false,
+    kwargs: exe_kwargs)
+  test(test_name, exe, kwargs: test_kwargs)
+endforeach