[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>

CMakeLists.txt

@@ -82,6 +82,23 @@ file(MAKE_DIRECTORY "${PROJECT_SOURCE_DIR}/scratch")
 # Python for shader compilation scripts
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
+# Generate build-tree version.h via xenia-build.py's generate_version_h()
+# (normally invoked by `xb premake`/`xb build`; CMake-direct flows need it too).
+execute_process(
+  COMMAND ${Python3_EXECUTABLE} -c
+    "import importlib.util,sys; \
+     spec=importlib.util.spec_from_file_location('xb',r'${PROJECT_SOURCE_DIR}/xenia-build.py'); \
+     m=importlib.util.module_from_spec(spec); spec.loader.exec_module(m); \
+     m.generate_version_h(r'${CMAKE_BINARY_DIR}')"
+  WORKING_DIRECTORY "${PROJECT_SOURCE_DIR}"
+  RESULT_VARIABLE _xenia_version_rc
+)
+if(NOT _xenia_version_rc EQUAL 0)
+  message(WARNING "version.h generation failed (rc=${_xenia_version_rc}); writing stub.")
+  file(WRITE "${CMAKE_BINARY_DIR}/version.h"
+    "#ifndef GENERATED_VERSION_H_\n#define GENERATED_VERSION_H_\n#define XE_BUILD_BRANCH \"unknown\"\n#define XE_BUILD_COMMIT \"unknown\"\n#define XE_BUILD_COMMIT_SHORT \"unknown\"\n#define XE_BUILD_DATE __DATE__\n#endif\n")
+endif()
+
 # Include helpers
 include(cmake/XeniaHelpers.cmake)
 
@@ -146,8 +163,14 @@ if(MSVC)
 
   # --- Per-configuration MSVC flags ---
   # Checked
-  string(APPEND CMAKE_C_FLAGS_CHECKED " /RTCsu /fsanitize=address")
-  string(APPEND CMAKE_CXX_FLAGS_CHECKED " /RTCsu /fsanitize=address")
+  if(CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"
+     AND NOT CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    string(APPEND CMAKE_C_FLAGS_CHECKED " /RTCsu /fsanitize=address")
+    string(APPEND CMAKE_CXX_FLAGS_CHECKED " /RTCsu /fsanitize=address")
+  else()
+    string(APPEND CMAKE_C_FLAGS_CHECKED " /fsanitize=address")
+    string(APPEND CMAKE_CXX_FLAGS_CHECKED " /fsanitize=address")
+  endif()
   string(APPEND CMAKE_EXE_LINKER_FLAGS_CHECKED " /INCREMENTAL:NO")
   add_compile_definitions($<$<CONFIG:Checked>:DEBUG>)