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/meson_options.txt

@@ -0,0 +1,22 @@
+# Optimizations
+option('fixed-point', type : 'boolean', value : false, description : 'Compile without floating point (for machines without a fast enough FPU')
+option('fixed-point-debug', type : 'boolean', value : false, description : 'Debug fixed-point implementation')
+option('float-api', type : 'boolean', value : true, description : 'Compile with or without the floating point API (for machines with no float library')
+option('float-approx', type : 'boolean', value : false, description : 'Enable fast approximations for floating point (not supported on all platforms)')
+option('rtcd', type : 'feature', value : 'auto', description : 'Run-time CPU capabilities detection')
+option('asm', type : 'feature', value : 'auto', description : 'Assembly optimizations for ARM (fixed-point)')
+option('intrinsics', type : 'feature', value : 'auto', description : 'Intrinsics optimizations for ARM NEON or x86')
+
+option('custom-modes', type : 'boolean', value : false, description : 'Enable non-Opus modes, e.g. 44.1 kHz & 2^n frames')
+option('extra-programs', type : 'feature', value : 'auto', description : 'Extra programs (demo and tests)')
+option('assertions', type : 'boolean', value : false, description : 'Additional software error checking')
+option('hardening', type : 'boolean', value : true, description : 'Run-time checks that are cheap and safe for use in production')
+option('fuzzing', type : 'boolean', value : false, description : 'Causes the encoder to make random decisions')
+option('check-asm', type : 'boolean', value : false, description : 'Run bit-exactness checks between optimized and c implementations')
+
+# common feature options
+option('tests', type : 'feature', value : 'auto', description : 'Build tests')
+option('docs', type: 'feature', value: 'auto', description: 'Build API documentation')
+
+# other options
+option('docdir', type: 'string', value: 'doc/opus', description: 'Directory to install documentation into (default: DATADIR/doc/opus')