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.

assets/patches/Common7/Tools/VsDevCmd.bat.patch

@@ -0,0 +1,18 @@
+msvc-go-wine: skip VsDevCmd's telemetry upload.
+
+There's no telemetry endpoint reachable (or wanted) under Wine. Rather than
+touching the conditional logic below, just set the opt-out switch the
+script already checks for on its own.
+
+diff --git a/Common7/Tools/VsDevCmd.bat b/Common7/Tools/VsDevCmd.bat
+index f925e95..f3710ad 100644
+--- a/Common7/Tools/VsDevCmd.bat
++++ b/Common7/Tools/VsDevCmd.bat
+@@ -247,6 +247,7 @@ exit /B 0
+ :end
+ 
+ @REM Send Telemetry if user's VS is opted-in
++set "VSCMD_SKIP_SENDTELEMETRY=1"
+ if "%VSCMD_SKIP_SENDTELEMETRY%"=="" (
+     if "%VSCMD_DEBUG%" NEQ "" (
+         @echo [DEBUG:%~nx0] Sending telemetry