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/fixed/schur_FIX.c

@@ -0,0 +1,107 @@
+/***********************************************************************
+Copyright (c) 2006-2011, Skype Limited. All rights reserved.
+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 "SigProc_FIX.h"
+
+/* Faster than schur64(), but much less accurate.                       */
+/* uses SMLAWB(), requiring armv5E and higher.                          */
+opus_int32 silk_schur(                              /* O    Returns residual energy                                     */
+    opus_int16                  *rc_Q15,            /* O    reflection coefficients [order] Q15                         */
+    const opus_int32            *c,                 /* I    correlations [order+1]                                      */
+    const opus_int32            order               /* I    prediction order                                            */
+)
+{
+    opus_int        k, n, lz;
+    opus_int32    C[ SILK_MAX_ORDER_LPC + 1 ][ 2 ];
+    opus_int32    Ctmp1, Ctmp2, rc_tmp_Q15;
+
+    celt_assert( order >= 0 && order <= SILK_MAX_ORDER_LPC );
+
+    /* Get number of leading zeros */
+    lz = silk_CLZ32( c[ 0 ] );
+
+    /* Copy correlations and adjust level to Q30 */
+    k = 0;
+    if( lz < 2 ) {
+        /* lz must be 1, so shift one to the right */
+        do {
+            C[ k ][ 0 ] = C[ k ][ 1 ] = silk_RSHIFT( c[ k ], 1 );
+        } while( ++k <= order );
+    } else if( lz > 2 ) {
+        /* Shift to the left */
+        lz -= 2;
+        do {
+            C[ k ][ 0 ] = C[ k ][ 1 ] = silk_LSHIFT( c[ k ], lz );
+        } while( ++k <= order );
+    } else {
+        /* No need to shift */
+        do {
+            C[ k ][ 0 ] = C[ k ][ 1 ] = c[ k ];
+        } while( ++k <= order );
+    }
+
+    for( k = 0; k < order; k++ ) {
+        /* Check that we won't be getting an unstable rc, otherwise stop here. */
+        if (silk_abs_int32(C[ k + 1 ][ 0 ]) >= C[ 0 ][ 1 ]) {
+           if ( C[ k + 1 ][ 0 ] > 0 ) {
+              rc_Q15[ k ] = -SILK_FIX_CONST( .99f, 15 );
+           } else {
+              rc_Q15[ k ] = SILK_FIX_CONST( .99f, 15 );
+           }
+           k++;
+           break;
+        }
+
+        /* Get reflection coefficient */
+        rc_tmp_Q15 = -silk_DIV32_16( C[ k + 1 ][ 0 ], silk_max_32( silk_RSHIFT( C[ 0 ][ 1 ], 15 ), 1 ) );
+
+        /* Clip (shouldn't happen for properly conditioned inputs) */
+        rc_tmp_Q15 = silk_SAT16( rc_tmp_Q15 );
+
+        /* Store */
+        rc_Q15[ k ] = (opus_int16)rc_tmp_Q15;
+
+        /* Update correlations */
+        for( n = 0; n < order - k; n++ ) {
+            Ctmp1 = C[ n + k + 1 ][ 0 ];
+            Ctmp2 = C[ n ][ 1 ];
+            C[ n + k + 1 ][ 0 ] = silk_SMLAWB( Ctmp1, silk_LSHIFT( Ctmp2, 1 ), rc_tmp_Q15 );
+            C[ n ][ 1 ]         = silk_SMLAWB( Ctmp2, silk_LSHIFT( Ctmp1, 1 ), rc_tmp_Q15 );
+        }
+    }
+
+    for(; k < order; k++ ) {
+       rc_Q15[ k ] = 0;
+    }
+
+    /* return residual energy */
+    return silk_max_32( 1, C[ 0 ][ 1 ] );
+}