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.

internal/wrapper/native.go

@@ -0,0 +1,39 @@
+package wrapper
+
+import (
+	"os"
+	"os/exec"
+)
+
+// runNative handles the two tool names that need no Wine/MSVC install at
+// all: `cmd` (strips a leading "//c" and execs the rest) and `findstr`
+// (delegates to grep).
+func runNative(tool string, args []string) int {
+	switch tool {
+	case "cmd":
+		if len(args) > 0 && args[0] == "//c" {
+			args = args[1:]
+		}
+		return execInherit(args)
+	case "findstr":
+		return execInherit(append([]string{"grep"}, args...))
+	}
+	return 127
+}
+
+func execInherit(args []string) int {
+	if len(args) == 0 {
+		return 0
+	}
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			return exitErr.ExitCode()
+		}
+		return 127
+	}
+	return 0
+}