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/README.draft

@@ -0,0 +1,54 @@
+To build this source code, simply type:
+
+% make
+
+If this does not work, or if you want to change the default configuration
+(e.g., to compile for a fixed-point architecture), simply edit the options
+in the Makefile.
+
+An up-to-date implementation conforming to this standard is available in a
+Git repository at https://gitlab.xiph.org/xiph/opus.git or on a website at:
+https://opus-codec.org/
+However, although that implementation is expected to remain conformant
+with the standard, it is the code in this RFC that shall remain normative.
+To build from the git repository instead of using this RFC, follow these
+steps:
+
+1) Clone the repository (latest implementation of this standard at the time
+of publication)
+
+% git clone https://gitlab.xiph.org/xiph/opus.git
+% cd opus
+
+2) Compile
+
+% ./autogen.sh
+% ./configure
+% make
+
+Once you have compiled the codec, there will be a opus_demo executable in
+the top directory.
+
+Usage: opus_demo [-e] <application> <sampling rate (Hz)> <channels (1/2)>
+         <bits per second> [options] <input> <output>
+       opus_demo -d <sampling rate (Hz)> <channels (1/2)> [options]
+         <input> <output>
+
+mode: voip | audio | restricted-lowdelay
+options:
+-e                   : only runs the encoder (output the bit-stream)
+-d                   : only runs the decoder (reads the bit-stream as input)
+-cbr                 : enable constant bitrate; default: variable bitrate
+-cvbr                : enable constrained variable bitrate; default: unconstrained
+-bandwidth <NB|MB|WB|SWB|FB> : audio bandwidth (from narrowband to fullband);
+                               default: sampling rate
+-framesize <2.5|5|10|20|40|60> : frame size in ms; default: 20
+-max_payload <bytes> : maximum payload size in bytes, default: 1024
+-complexity <comp>   : complexity, 0 (lowest) ... 10 (highest); default: 10
+-inbandfec           : enable SILK inband FEC
+-forcemono           : force mono encoding, even for stereo input
+-dtx                 : enable SILK DTX
+-loss <perc>         : simulate packet loss, in percent (0-100); default: 0
+
+input and output are little endian signed 16-bit PCM files or opus bitstreams
+with simple opus_demo proprietary framing.