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>

Cargo.toml

@@ -76,3 +76,16 @@ opt-level = 3
 opt-level = 3
 [profile.dev.package.wzp-fec]
 opt-level = 3
+
+# Vendored audiopus_sys with a patched opus/CMakeLists.txt that distinguishes
+# real cl.exe (MSVC) from clang-cl (used by cargo-xwin for Windows cross-
+# compiles). Upstream libopus 1.3.1 gates its `-msse4.1` per-file compile
+# flags on `if(NOT MSVC)`, which is false under clang-cl because CMake sets
+# MSVC=1 for both compilers — resulting in SSE4.1 source files compiled
+# without the required target feature and hard failures in silk/NSQ_sse4_1.c.
+# The vendored copy introduces an `MSVC_CL` var (true only for real cl.exe)
+# and flips the SIMD guards to use it, restoring per-file SIMD flags for
+# clang-cl. See vendor/audiopus_sys/opus/CMakeLists.txt for the full diff
+# and rationale, plus xiph/opus#256 / xiph/opus PR #257 upstream.
+[patch.crates-io]
+audiopus_sys = { path = "vendor/audiopus_sys" }