[Build] Linux→Windows cross-compile toolchain (clang-cl + xwin)

New CMake preset `cross-win-clangcl` for Ninja Multi-Config on
non-Windows hosts. Toolchain: clang-cl (MSVC-ABI), lld-link, xwin SDK/CRT
splat. Shader compilation routes through wine fxc.exe with FXC_PATH
env forwarding and unix→Windows path translation via `winepath -w`.
Plus per-file build fixes (constexpr→const, llvm-rc forward-slash,
zlib-ng AVX guard, /RTCsu MSVC-only). third_party/snappy bumped to
fabi/sylpheed-crossbuild for the target-aware POSIX-gate header.

Produces `xenia_canary.exe` (Debug MSVC) suitable for use as the
audit oracle under Wine for xenia-rs work. See
docs/CROSS_BUILD_SETUP.md for the full reproduction recipe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/build/compile_shader_dxbc.py

@@ -73,6 +73,16 @@ def main():
 
     # Start with base command — use wine on non-Windows platforms.
     if sys.platform != "win32":
+        def _wineify(p):
+            try:
+                out = subprocess.check_output(["winepath", "-w", p],
+                                              stderr=subprocess.DEVNULL)
+                return out.decode("utf-8", "replace").strip()
+            except (OSError, subprocess.CalledProcessError):
+                return p
+        input_path = _wineify(input_path)
+        output_path = _wineify(output_path)
+        src_dir = _wineify(src_dir)
         compiler_args = ["wine", fxc]
     else:
         compiler_args = [fxc]