[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>

src/xenia/app/main_resources.rc

@@ -1,3 +1,3 @@
 //{{NO_DEPENDENCIES}}
 
-MAINICON               ICON                    "..\\..\\..\\assets\\icon\\icon.ico"
+MAINICON               ICON                    "../../../assets/icon/icon.ico"

src/xenia/base/mapped_memory_win.cc

@@ -26,8 +26,9 @@ namespace xe {
 class Win32MappedMemory : public MappedMemory {
  public:
   // CreateFile returns INVALID_HANDLE_VALUE in case of failure.
-  // chrispy: made inline const to get around clang error
-  static inline constexpr HANDLE kFileHandleInvalid = INVALID_HANDLE_VALUE;
+  // INVALID_HANDLE_VALUE expands to a reinterpret_cast which MSVC accepts
+  // in constexpr as an extension; clang-cl rejects it per C++ standard.
+  static inline const HANDLE kFileHandleInvalid = INVALID_HANDLE_VALUE;
   // CreateFileMapping returns nullptr in case of failure.
   static constexpr HANDLE kMappingHandleInvalid = nullptr;