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.

LICENSE.txt

@@ -0,0 +1,25 @@
+MIT License
+
+Copyright (c) 2026 Cheviiot
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+This license covers msvc-go-wine's own source code. It does not cover the
+Microsoft Visual C++ Build Tools / Windows SDK that `msvc-go-wine download`
+fetches and unpacks, which remain governed by Microsoft's own license terms.