[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>

cmake/XeniaHelpers.cmake

@@ -192,6 +192,12 @@ function(xe_shader_rules_dxbc target shader_dir)
   set(_valid_stages vs hs ds gs ps cs)
   set(_commands)
   set(_bytecode_dir "${shader_dir}/bytecode/d3d12_5_1")
+  # Propagate FXC_PATH from the configure-time env so ninja-spawned python
+  # subprocesses can find fxc.exe (CMake `set(ENV{...})` doesn't reach build).
+  set(_env_prefix "")
+  if(DEFINED ENV{FXC_PATH})
+    set(_env_prefix ${CMAKE_COMMAND} -E env "FXC_PATH=$ENV{FXC_PATH}")
+  endif()
   list(APPEND _commands COMMAND ${CMAKE_COMMAND} -E make_directory "${_bytecode_dir}")
   foreach(src ${_sources})
     get_filename_component(_name ${src} NAME)
@@ -206,7 +212,7 @@ function(xe_shader_rules_dxbc target shader_dir)
     if(NOT _stage IN_LIST _valid_stages)
       continue()
     endif()
-    list(APPEND _commands COMMAND ${Python3_EXECUTABLE} "${_script}" "${src}" "${_bytecode_dir}/${_id}.h")
+    list(APPEND _commands COMMAND ${_env_prefix} ${Python3_EXECUTABLE} "${_script}" "${src}" "${_bytecode_dir}/${_id}.h")
   endforeach()
   add_custom_command(
     OUTPUT "${_stamp}"