Initial implementation of msvc-go-wine

A single-binary Go tool for cross compiling with the real MSVC toolchain
on Linux via Wine. Behaves as cl/link/lib/rc/midl/mt/dumpbin/msbuild/
nmake/ml/ml64/armasm/armasm64/cmd/findstr depending on the name it's
invoked as, plus download/install/env/version management subcommands.

- download: fetches the MSVC/WinSDK installer manifest, resolves package
  selection and dependencies, downloads and verifies payloads, unpacks
  VSIX/MSI packages, and applies a handful of compatibility patches so
  VsDevCmd.bat and MSBuild's SDK detection work without a Windows
  Registry (which doesn't exist under Wine).
- install: locates the installed toolchain/SDK versions, normalizes
  header/library name casing, lays out per-architecture tool symlinks
  with an env.json config each, and compiles a small native launcher
  (toolrelay.exe) that lets mt.exe's CMake-compatibility exit code
  survive Wine's own exit-code truncation.
- The wrapper runtime rewrites absolute unix paths in tool arguments into
  Wine's z:\... form, runs the real .exe under wine, and rewrites the
  tool's output back to plain unix paths.

Verified end-to-end against a real MSVC/WinSDK download: cl, link, mt and
the resulting hello.exe all work under Wine, including through the
toolrelay.exe relay path and with paths containing non-ASCII characters.

Offline unit tests cover the wrapper's path-rewrite/output-filter logic,
install-time header lowercasing, and download package-selection/
dependency-resolution.

cmd/msvc-go-wine/env.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/Cheviiot/msvc-go-wine/internal/wineenv"
+)
+
+// runEnv prints shell `export` statements for INCLUDE/LIB (converted from
+// wine's "z:\..." notation to plain unix paths) and TARGET_TRIPLE, for
+// driving clang-cl/lld-link directly without Wine.
+// Usage: eval "$(msvc-go-wine env --bin <dest>/bin/<arch>)"
+func runEnv(args []string) int {
+	fs := flag.NewFlagSet("env", flag.ContinueOnError)
+	bin := fs.String("bin", "", "the <dest>/bin/<arch> directory produced by `msvc-go-wine install`")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+	if *bin == "" {
+		fmt.Fprintln(os.Stderr, "usage: msvc-go-wine env --bin <dest>/bin/<arch>")
+		return 1
+	}
+
+	cfg, err := wineenv.Load(*bin)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "msvc-go-wine env:", err)
+		return 1
+	}
+	baseUnix, err := wineenv.FindBaseUnix(*bin)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "msvc-go-wine env:", err)
+		return 1
+	}
+	paths := wineenv.NewPaths(cfg, baseUnix)
+
+	triple, ok := targetTriples[cfg.Arch]
+	if !ok {
+		fmt.Fprintf(os.Stderr, "msvc-go-wine env: unknown arch %q\n", cfg.Arch)
+		return 1
+	}
+
+	fmt.Printf("export INCLUDE=%q\n", toUnixPathList(paths.Include))
+	fmt.Printf("export LIB=%q\n", toUnixPathList(paths.Lib))
+	fmt.Printf("export TARGET_TRIPLE=%q\n", triple)
+	return 0
+}
+
+var targetTriples = map[string]string{
+	"x86":   "i686-windows-msvc",
+	"x64":   "x86_64-windows-msvc",
+	"arm":   "armv7-windows-msvc",
+	"arm64": "aarch64-windows-msvc",
+}
+
+// toUnixPathList does a blanket removal of the "z:" drive prefix and
+// backslash->slash conversion across the whole semicolon-joined path list.
+func toUnixPathList(s string) string {
+	s = strings.ReplaceAll(s, "z:", "")
+	s = strings.ReplaceAll(s, `\`, "/")
+	return s
+}