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/silk/meson.build

@@ -0,0 +1,53 @@
+silk_sources = sources['SILK_SOURCES']
+
+silk_sources_sse4_1 = sources['SILK_SOURCES_SSE4_1']
+
+silk_sources_neon_intr = sources['SILK_SOURCES_ARM_NEON_INTR']
+
+silk_sources_fixed_neon_intr = sources['SILK_SOURCES_FIXED_ARM_NEON_INTR']
+
+silk_sources_fixed = sources['SILK_SOURCES_FIXED']
+
+silk_sources_fixed_sse4_1 = sources['SILK_SOURCES_FIXED_SSE4_1']
+
+silk_sources_float = sources['SILK_SOURCES_FLOAT']
+
+if opt_fixed_point
+  silk_sources += silk_sources_fixed
+else
+  silk_sources += silk_sources_float
+endif
+
+silk_includes = [opus_includes, include_directories('float', 'fixed')]
+silk_static_libs = []
+
+foreach intr_name : ['sse4_1', 'neon_intr']
+  have_intr = get_variable('have_' + intr_name)
+  if not have_intr
+    continue
+  endif
+
+  intr_sources = get_variable('silk_sources_' + intr_name)
+  if opt_fixed_point
+    intr_sources += get_variable('silk_sources_fixed_' + intr_name)
+  endif
+
+  intr_args = get_variable('opus_@0@_args'.format(intr_name), [])
+  silk_static_libs += static_library('silk_' + intr_name, intr_sources,
+      c_args: intr_args,
+      include_directories: silk_includes,
+      install: false)
+endforeach
+
+silk_c_args = []
+if host_machine.system() == 'windows'
+  silk_c_args += ['-DDLL_EXPORT']
+endif
+
+silk_lib = static_library('opus-silk',
+  silk_sources,
+  c_args: silk_c_args,
+  include_directories: silk_includes,
+  link_whole: silk_static_libs,
+  dependencies: libm,
+  install: false)