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/tests/meson.build

@@ -0,0 +1,8 @@
+exe = executable('test_unit_LPC_inv_pred_gain',
+  'test_unit_LPC_inv_pred_gain.c', '../LPC_inv_pred_gain.c',
+  include_directories: opus_includes,
+  link_with: [celt_lib, celt_static_libs, silk_lib, silk_static_libs],
+  dependencies: libm,
+  install: false)
+
+test(test_name, exe)

vendor/audiopus_sys/opus/silk/tests/test_unit_LPC_inv_pred_gain.c

@@ -0,0 +1,129 @@
+/***********************************************************************
+Copyright (c) 2017 Google Inc., Jean-Marc Valin
+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.
+- Neither the name of Internet Society, IETF or IETF Trust, nor the
+names of specific contributors, may be used to endorse or promote
+products derived from this software without specific prior written
+permission.
+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.
+***********************************************************************/
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "celt/stack_alloc.h"
+#include "cpu_support.h"
+#include "SigProc_FIX.h"
+
+/* Computes the impulse response of the filter so we
+   can catch filters that are definitely unstable. Some
+   unstable filters may be classified as stable, but not
+   the other way around. */
+int check_stability(opus_int16 *A_Q12, int order) {
+    int i;
+    int j;
+    int sum_a, sum_abs_a;
+    sum_a = sum_abs_a = 0;
+    for( j = 0; j < order; j++ ) {
+        sum_a += A_Q12[ j ];
+        sum_abs_a += silk_abs( A_Q12[ j ] );
+    }
+    /* Check DC stability. */
+    if( sum_a >= 4096 ) {
+        return 0;
+    }
+    /* If the sum of absolute values is less than 1, the filter
+       has to be stable. */
+    if( sum_abs_a < 4096 ) {
+        return 1;
+    }
+    double y[SILK_MAX_ORDER_LPC] = {0};
+    y[0] = 1;
+    for( i = 0; i < 10000; i++ ) {
+        double sum = 0;
+        for( j = 0; j < order; j++ ) {
+            sum += y[ j ]*A_Q12[ j ];
+        }
+        for( j = order - 1; j > 0; j-- ) {
+            y[ j ] = y[ j - 1 ];
+        }
+        y[ 0 ] = sum*(1./4096);
+        /* If impulse response reaches +/- 10000, the filter
+           is definitely unstable. */
+        if( !(y[ 0 ] < 10000 && y[ 0 ] > -10000) ) {
+            return 0;
+        }
+        /* Test every 8 sample for low amplitude. */
+        if( ( i & 0x7 ) == 0 ) {
+            double amp = 0;
+            for( j = 0; j < order; j++ ) {
+                amp += fabs(y[j]);
+            }
+            if( amp < 0.00001 ) {
+                return 1;
+            }
+        }
+    }
+    return 1;
+}
+
+int main(void) {
+    const int arch = opus_select_arch();
+    /* Set to 10000 so all branches in C function are triggered */
+    const int loop_num = 10000;
+    int count = 0;
+    ALLOC_STACK;
+
+    /* FIXME: Make the seed random (with option to set it explicitly)
+       so we get wider coverage. */
+    srand(0);
+
+    printf("Testing silk_LPC_inverse_pred_gain() optimization ...\n");
+    for( count = 0; count < loop_num; count++ ) {
+        unsigned int i;
+        opus_int     order;
+        unsigned int shift;
+        opus_int16   A_Q12[ SILK_MAX_ORDER_LPC ];
+        opus_int32 gain;
+
+        for( order = 2; order <= SILK_MAX_ORDER_LPC; order += 2 ) { /* order must be even. */
+            for( shift = 0; shift < 16; shift++ ) { /* Different dynamic range. */
+                for( i = 0; i < SILK_MAX_ORDER_LPC; i++ ) {
+                    A_Q12[i] = ((opus_int16)rand()) >> shift;
+                }
+                gain = silk_LPC_inverse_pred_gain(A_Q12, order, arch);
+                /* Look for filters that silk_LPC_inverse_pred_gain() thinks are
+                   stable but definitely aren't. */
+                if( gain != 0 && !check_stability(A_Q12, order) ) {
+                    fprintf(stderr, "**Loop %4d failed!**\n", count);
+                    return 1;
+                }
+            }
+        }
+        if( !(count % 500) ) {
+            printf("Loop %4d passed\n", count);
+        }
+    }
+    printf("silk_LPC_inverse_pred_gain() optimization passed\n");
+    return 0;
+}