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/tests/test_opus_padding.c

@@ -0,0 +1,93 @@
+/* Copyright (c) 2012 Xiph.Org Foundation
+   Written by Jüri Aedla and Ralph Giles */
+/*
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions
+   are met:
+
+   - Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+
+   - Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
+   OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+/* Check for overflow in reading the padding length.
+ * http://lists.xiph.org/pipermail/opus/2012-November/001834.html
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "opus.h"
+#include "test_opus_common.h"
+
+#define PACKETSIZE 16909318
+#define CHANNELS 2
+#define FRAMESIZE 5760
+
+int test_overflow(void)
+{
+  OpusDecoder *decoder;
+  int result;
+  int error;
+
+  unsigned char *in = malloc(PACKETSIZE);
+  opus_int16 *out = malloc(FRAMESIZE*CHANNELS*sizeof(*out));
+
+  fprintf(stderr, "  Checking for padding overflow... ");
+  if (!in || !out) {
+    fprintf(stderr, "FAIL (out of memory)\n");
+    return -1;
+  }
+  in[0] = 0xff;
+  in[1] = 0x41;
+  memset(in + 2, 0xff, PACKETSIZE - 3);
+  in[PACKETSIZE-1] = 0x0b;
+
+  decoder = opus_decoder_create(48000, CHANNELS, &error);
+  result = opus_decode(decoder, in, PACKETSIZE, out, FRAMESIZE, 0);
+  opus_decoder_destroy(decoder);
+
+  free(in);
+  free(out);
+
+  if (result != OPUS_INVALID_PACKET) {
+    fprintf(stderr, "FAIL!\n");
+    test_failed();
+  }
+
+  fprintf(stderr, "OK.\n");
+
+  return 1;
+}
+
+int main(void)
+{
+  const char *oversion;
+  int tests = 0;;
+
+  iseed = 0;
+  oversion = opus_get_version_string();
+  if (!oversion) test_failed();
+  fprintf(stderr, "Testing %s padding.\n", oversion);
+
+  tests += test_overflow();
+
+  fprintf(stderr, "All padding tests passed.\n");
+
+  return 0;
+}