[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/toolchains/xwin-case-symlinks.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+"""Scan xenia source for #include <...> directives and ensure case-aliases
+exist in xwin's flat SDK/CRT include dirs. Idempotent — safe to re-run."""
+from __future__ import annotations
+import os, re, sys
+from pathlib import Path
+
+INCLUDE_RE = re.compile(r'^\s*#\s*include\s*<([A-Za-z][A-Za-z0-9_/.\-]*\.h)>',
+                        re.MULTILINE)
+SOURCE_ROOTS = ["src", "third_party/SDL2/src", "third_party/SDL2/include",
+                "third_party/discord-rpc/src", "third_party/fmt",
+                "third_party/imgui"]
+SOURCE_EXTS = {".h", ".hpp", ".inc", ".c", ".cc", ".cpp", ".cxx"}
+
+def collect_includes(repo_root: Path) -> set[str]:
+    seen: set[str] = set()
+    for root in SOURCE_ROOTS:
+        base = repo_root / root
+        if not base.exists(): continue
+        for path in base.rglob("*"):
+            if path.suffix.lower() not in SOURCE_EXTS: continue
+            try: txt = path.read_text(encoding="utf-8", errors="ignore")
+            except OSError: continue
+            for m in INCLUDE_RE.finditer(txt):
+                seen.add(m.group(1))
+    return seen
+
+def fix_dir(target_dir: Path, want: set[str]) -> int:
+    if not target_dir.is_dir(): return 0
+    idx = {e.name.lower(): e.name for e in target_dir.iterdir()}
+    created = 0
+    for name in want:
+        if "/" in name: continue
+        actual = idx.get(name.lower())
+        if actual is None or actual == name: continue
+        link = target_dir / name
+        if link.exists() or link.is_symlink(): continue
+        try:
+            link.symlink_to(actual)
+            created += 1
+        except OSError: pass
+    return created
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        sys.exit("usage: xwin-case-symlinks.py <xenia-canary-root> <xwin-splat-dir>")
+    repo = Path(sys.argv[1]).resolve()
+    xwin = Path(sys.argv[2]).resolve()
+    want = collect_includes(repo)
+    total = 0
+    for d in (xwin/"crt/include", xwin/"sdk/include/ucrt",
+              xwin/"sdk/include/um", xwin/"sdk/include/shared",
+              xwin/"sdk/include/winrt"):
+        total += fix_dir(d, want)
+    print(f"Created {total} case-symlinks across {len(want)} unique includes.")