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/meson/read-sources-list.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+#
+# opus/read-sources-list.py
+#
+# Parses .mk files and extracts list of source files.
+# Prints one line per source file list, with filenames space-separated.
+
+import sys
+
+if len(sys.argv) < 2:
+  sys.exit('Usage: {} sources_foo.mk [sources_bar.mk...]'.format(sys.argv[0]))
+
+for input_fn in sys.argv[1:]:
+  with open(input_fn, 'r', encoding='utf8') as f:
+    text = f.read()
+    text = text.replace('\\\n', '')
+
+    # Remove empty lines
+    lines = [line for line in text.split('\n') if line.strip()]
+
+    # Print SOURCES_XYZ = file1.c file2.c
+    for line in lines:
+      values = line.strip().split('=', maxsplit=2)
+      if len(values) != 2:
+        raise RuntimeError('Unable to parse line "{}" from file "{}"'.format(line, input_fn))
+      var, files = values
+      sources_list = [f for f in files.split(' ') if f]
+      print(var.strip(), '=', ' '.join(sources_list))