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/doc/opus_in_isobmff.css

@@ -0,0 +1,60 @@
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+{
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+}
+
+/* Boxes */
+.pre
+{
+	white-space: pre;           /* CSS 2.0 */
+	white-space: pre-wrap;      /* CSS 2.1 */
+	white-space: -pre-wrap;     /* Opera 4-6 */
+	white-space: -o-pre-wrap;   /* Opera 7 */
+	white-space: -moz-pre-wrap; /* Mozilla */
+	white-space: -hp-pre-wrap;  /* HP Printers */
+	word-wrap  : break-word;    /* IE 5+ */
+}
+
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+{
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+    margin           : 2px 50px 2px 2px;
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+    background-color : #666666;
+    white-space      : pre;
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+    color            : #C0C0C0;
+    font-size        : 50pt;
+    font-style       : italic;
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+{
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