[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>

CMakePresets.json

@@ -39,6 +39,22 @@
       "cacheVariables": {
         "CMAKE_SYSTEM_PROCESSOR": "ARM64"
       }
+    },
+    {
+      "name": "cross-win-clangcl",
+      "displayName": "Cross (Linux→Win MSVC) clang-cl + xwin",
+      "generator": "Ninja Multi-Config",
+      "binaryDir": "${sourceDir}/build-cross",
+      "toolchainFile": "${sourceDir}/cmake/toolchains/linux-to-win-msvc.cmake",
+      "condition": {
+        "type": "notEquals",
+        "lhs": "${hostSystemName}",
+        "rhs": "Windows"
+      },
+      "cacheVariables": {
+        "XWIN_DIR":  "$env{HOME}/.xwin/splat",
+        "FXC_PATH":  "$env{HOME}/.local/share/xenia-cross/fxc/fxc.exe"
+      }
     }
   ],
   "buildPresets": [
@@ -89,6 +105,9 @@
       "name": "vs-arm64-checked",
       "configurePreset": "vs-arm64",
       "configuration": "Checked"
-    }
+    },
+    { "name": "cross-debug",   "configurePreset": "cross-win-clangcl", "configuration": "Debug" },
+    { "name": "cross-release", "configurePreset": "cross-win-clangcl", "configuration": "Release" },
+    { "name": "cross-checked", "configurePreset": "cross-win-clangcl", "configuration": "Checked" }
   ]
 }