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/clpost.go

@@ -0,0 +1,39 @@
+package wrapper
+
+import (
+	"os"
+	"strings"
+)
+
+// clPostProcess mirrors cl's post-run fixup for `/P /Fi<file>` (preprocess-
+// to-file): the generated file still has CRLF endings and z:-prefixed
+// #line directives, so run it through the same line-directive rewrite used
+// for stdout.
+func clPostProcess(origArgs []string) {
+	var hasP bool
+	var fiFile string
+	for _, a := range origArgs {
+		switch {
+		case a == "-P" || a == "/P":
+			hasP = true
+		case strings.HasPrefix(a, "-Fi") || strings.HasPrefix(a, "/Fi"):
+			fiFile = a[3:]
+		}
+	}
+	if !hasP || fiFile == "" {
+		return
+	}
+	data, err := os.ReadFile(fiFile)
+	if err != nil {
+		return
+	}
+	lines := strings.Split(string(data), "\n")
+	for i, line := range lines {
+		line = stripCR(line)
+		if reLineDirective.MatchString(line) {
+			line = strings.ReplaceAll(stripFirstZDrive(line), `\\`, `/`)
+		}
+		lines[i] = line
+	}
+	_ = os.WriteFile(fiFile, []byte(strings.Join(lines, "\n")), 0o644)
+}