windows/build-msi.sh: cross-build the MSI on Linux via wixl + mingw-w64

Discovered building v1.1.0 that the MSI can be produced entirely on
Linux — no Windows VM, no manual WiX install, no GUI babysitting:

  apt install wixl unzip gcc-mingw-w64-x86-64 binutils-mingw-w64-x86-64 \
              mingw-w64-x86-64-dev curl

The new build-msi.sh script:
  1. Runs sync.sh to refresh pkg/{lmcp,server,json}.lua from root.
  2. Downloads Lua 5.4.2 Win64 binaries from LuaBinaries (Tools +
     Library zips — interpreter + headers + import lib).
  3. Cross-compiles LuaSocket 3.1.0 via x86_64-w64-mingw32-gcc
     (produces socket-3.0.0.dll + mime-1.0.3.dll for Win64).
  4. Stages pkg/lua/{lua.exe, lua54.dll, socket/, mime/, *.lua} per
     the WiX manifest layout.
  5. Invokes wixl on the lmcp.wxs manifest (with sed for the
     Windows backslash path separators → forward slashes).

Output: lmcp-<version>.msi. Version is read from lmcp.wxs
Version="…", so bump that before each release.

Cold build: ~30s. Warm cache: ~5s. The artifact contains all 17
files the WiX manifest expects, ProductVersion matches lmcp.wxs.

README updated to point at build-msi.sh as the recommended path;
the Windows-side candle/light recipe kept as an alternative.

Reproducibility note (deferred): the MSI is not yet bit-reproducible
across builds — file mtimes in the Lua binaries' zip propagate to
the cab inside the MSI. The debian/lmcp/build-deb.sh in marfrit-
packages uses SOURCE_DATE_EPOCH to fix this; same pattern would
apply here. Out of scope for the first cut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

windows/README.md

@@ -3,14 +3,30 @@
 This directory contains the WiX manifest and packaging files for the
 Windows MSI build of lmcp.
 
-## Workflow
+## Recommended: cross-build on Linux (one command)
 
 ```sh
-# 1. Pull the Lua + LuaSocket runtime into pkg/lua/ (one-time, see below).
-# 2. Sync the lmcp .lua sources from the root of the repo:
-./sync.sh
-# 3. Bump windows/lmcp.wxs `Version="…"` to match the release tag.
-# 4. Invoke WiX:
+./build-msi.sh /path/to/output/dir
+```
+
+Downloads Lua 5.4 Win64 binaries from LuaBinaries, cross-compiles
+LuaSocket via `mingw-w64`, stages `pkg/lua/`, and runs `wixl` to
+produce `lmcp-<version>.msi`. No Windows VM required.
+
+Prereqs on a Debian/Ubuntu builder:
+```sh
+sudo apt install wixl unzip gcc-mingw-w64-x86-64 \
+                 binutils-mingw-w64-x86-64 mingw-w64-x86-64-dev curl
+```
+
+Version comes from `lmcp.wxs` `Version="…"`. Bump that before
+building a release.
+
+## Alternative: build on Windows via WiX toolset
+
+```cmd
+sync.sh                                  REM see "tracked vs. generated"
+REM ensure pkg/lua/ has the runtime — see below
 candle.exe lmcp.wxs
 light.exe lmcp.wixobj -o lmcp-1.x.y.msi
 ```