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/README.md

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+[![ci-badge][]][ci] [![docs-badge][]][docs] [![rust version badge]][rust version link] [![crates.io version]][crates.io link]
+
+# About
+
+`audiopus_sys` is an FFI-Rust-binding to [`Opus`] version 1.3.
+
+Orginally, this sys-crate was made to empower the [`serenity`]-crate to build audio features on Windows, Linux, and Mac. However, it's not limited to that.
+
+Everyone is welcome to contribute,
+check out the [`CONTRIBUTING.md`](CONTRIBUTING.md) for further guidance.
+
+# Building
+
+## Requirements
+If you want to build Opus, you will need `cmake`.
+
+If you have `pkg-config`, it will attempt to use that before building.
+
+You can also link a pre-installed Opus, see [**Pre-installed Opus**](#Pre-installed-Opus)
+below.
+
+This crate provides a pre-built binding. In case you want to generate the
+binding yourself, you will need [`Clang`](https://rust-lang.github.io/rust-bindgen/requirements.html#clang),
+see [**Pre-installed Opus**](#Generating-The-Binding) below for further
+instructions.
+
+## Linking
+`audiopus_sys` links to Opus 1.3 and supports Windows, Linux, and MacOS
+By default, we statically link to Windows, MacOS, and if you use the
+`musl`-environment. We will link dynamically for Linux except when using
+mentioned `musl`.
+
+This can be altered by compiling with the `static` or `dynamic` feature having
+effects respective to their names. If both features are enabled,
+we will pick your system's default.
+
+Environment variables named `LIBOPUS_STATIC` or `OPUS_STATIC` will take
+precedence over features thus overriding the behaviour. The value of these
+environment variables have no influence of the result: If one of them is set,
+statically linking will be picked.
+
+## Pkg-Config
+By default, `audiopus_sys` will use `pkg-config` on Unix or GNU.
+Setting the environment variable `LIBOPUS_NO_PKG` or `OPUS_NO_PKG` will bypass
+probing for Opus via `pkg-config`.
+
+## Pre-installed Opus
+If you have Opus pre-installed, you can set `LIBOPUS_LIB_DIR` or
+`OPUS_LIB_DIR` to the directory containing Opus.
+
+Be aware that using an Opus other than version 1.3 may not work.
+
+# Generating The Binding
+If you want to generate the binding yourself, you can use the
+`generate_binding`-feature.
+
+Be aware, `bindgen` requires Clang and its `LIBCLANG_PATH`
+environment variable to be specified.
+
+# Installation
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+audiopus_sys = "0.2"
+```
+[`serenity`]: https://crates.io/crates/serenity
+
+[`Opus`]: https://www.opus-codec.org/
+
+[ci-badge]: https://img.shields.io/github/workflow/status/Lakelezz/audiopus_sys/CI?style=flat-square
+[ci]: https://github.com/Lakelezz/audiopus_sys/actions
+
+[docs-badge]: https://img.shields.io/badge/docs-online-5023dd.svg?style=flat-square&colorB=32b6b7
+[docs]: https://docs.rs/audiopus_sys
+
+[rust version badge]: https://img.shields.io/badge/rust-1.51+-93450a.svg?style=flat-square&colorB=ff9a0d
+[rust version link]: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html
+
+[crates.io link]: https://crates.io/crates/audiopus_sys
+[crates.io version]: https://img.shields.io/crates/v/audiopus_sys.svg?style=flat-square&colorB=b73732