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_headers.mk

@@ -0,0 +1,44 @@
+SILK_HEAD = \
+silk/debug.h \
+silk/control.h \
+silk/errors.h \
+silk/API.h \
+silk/typedef.h \
+silk/define.h \
+silk/main.h \
+silk/x86/main_sse.h \
+silk/PLC.h \
+silk/structs.h \
+silk/tables.h \
+silk/tuning_parameters.h \
+silk/Inlines.h \
+silk/MacroCount.h \
+silk/MacroDebug.h \
+silk/macros.h \
+silk/NSQ.h \
+silk/pitch_est_defines.h \
+silk/resampler_private.h \
+silk/resampler_rom.h \
+silk/resampler_structs.h \
+silk/SigProc_FIX.h \
+silk/x86/SigProc_FIX_sse.h \
+silk/arm/biquad_alt_arm.h \
+silk/arm/LPC_inv_pred_gain_arm.h \
+silk/arm/macros_armv4.h \
+silk/arm/macros_armv5e.h \
+silk/arm/macros_arm64.h \
+silk/arm/SigProc_FIX_armv4.h \
+silk/arm/SigProc_FIX_armv5e.h \
+silk/arm/NSQ_del_dec_arm.h \
+silk/arm/NSQ_neon.h \
+silk/fixed/main_FIX.h \
+silk/fixed/structs_FIX.h \
+silk/fixed/arm/warped_autocorrelation_FIX_arm.h \
+silk/fixed/mips/noise_shape_analysis_FIX_mipsr1.h \
+silk/fixed/mips/warped_autocorrelation_FIX_mipsr1.h \
+silk/float/main_FLP.h \
+silk/float/structs_FLP.h \
+silk/float/SigProc_FLP.h \
+silk/mips/macros_mipsr1.h \
+silk/mips/NSQ_del_dec_mipsr1.h \
+silk/mips/sigproc_fix_mipsr1.h