feat(gui): dockerized Windows cross-build (win-builder image + one-shot dist script) + GUI/backend scenario docs

cmd/gui/docker/win-builder/Dockerfile

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+# ModelRouter GUI Windows builder
+# Self-sufficient cross-compilation environment for the embedded llmsproxy core.
+# Produces llmsproxy.exe (Go, windows/amd64, luajit tag) + lua51.dll (LuaJIT mingw build)
+# inside a mounted workspace: cmd/gui/bin/{llmsproxy.exe,lua51.dll}
+#
+# Build:   docker build -t modelrouter/win-builder cmd/gui/docker/win-builder
+# Usage:   make gui-win-docker   (or cmd/gui/scripts/dist-win-docker.sh)
+#
+# LuaJIT source is fetched from gitcode.com (CN-reachable mirror) with github
+# fallback; pass --build-arg LUAJIT_REPO to override.
+
+FROM golang:1.25
+
+ARG LUAJIT_REPO=https://gitcode.com/openresty/luajit2.git
+
+RUN apt-get update -qq \
+ && DEBIAN_FRONTEND=noninteractive apt-get install -y -qq \
+      gcc-mingw-w64-x86-64 make ca-certificates git \
+ && rm -rf /var/lib/apt/lists/*
+
+# LuaJIT cross-compiled for Windows; keeps lua51.dll and the cgo import lib
+# (libluajit-5.1.dll.a -> libluajit-5.1.a in the mingw lib dir).
+# NOTE: $LUAJIT_REPO deliberately unquoted in "sh -c" — quoting the ARG would
+#       embed literal quotes into the URL.
+RUN git clone --depth 1 $LUAJIT_REPO /opt/luajit2 \
+ || git clone --depth 1 https://github.com/openresty/luajit2 /opt/luajit2 \
+ && cd /opt/luajit2 \
+ && make HOST_CC=gcc CROSS=x86_64-w64-mingw32- TARGET_SYS=Windows TARGET_XCFLAGS=-DLUAJIT_ENABLE_LUA52COMPAT \
+ && cp src/libluajit-5.1.dll.a /usr/x86_64-w64-mingw32/lib/libluajit-5.1.a
+
+# Go proxy mirror for builds behind GFW-ish networks; overridable at runtime.
+ENV GOPROXY=https://goproxy.cn,direct
+
+WORKDIR /workspace
+CMD ["/bin/bash"]