feat(server): complete phase 6 local verification

Dockerfile

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+# Local-only dedicated-server build and verification image. Pin the Godot
+# release family used by project.godot; no image is pushed by this repository.
+FROM --platform=linux/amd64 barichello/godot-ci:4.7.1 AS exporter
+WORKDIR /workspace
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends libfontconfig1 \
+ && rm -rf /var/lib/apt/lists/*
+COPY Game /workspace/Game
+# Godot dedicated exports disallow command-line scene overrides. Bake the
+# server scene into this export (the interactive project's source stays
+# unchanged), then generate the global-script/autoload metadata it needs.
+RUN sed -i 's|^run/main_scene=.*$|run/main_scene="res://scenes/server_boot.tscn"|' Game/project.godot \
+ && godot --headless --editor --path Game --import --quit \
+ && mkdir -p /opt/cosmic-clash \
+ && godot --headless --path Game --export-release "Linux Dedicated Server" /opt/cosmic-clash/CosmicClashServer.x86_64
+
+FROM --platform=linux/amd64 ubuntu:24.04 AS server
+RUN apt-get update && apt-get install -y --no-install-recommends libfontconfig1 libgl1 libstdc++6 && rm -rf /var/lib/apt/lists/*
+COPY --from=exporter /opt/cosmic-clash/ /opt/cosmic-clash/
+COPY deploy/cosmic-clash-server /opt/cosmic-clash/cosmic-clash-server
+RUN chmod 0755 /opt/cosmic-clash/cosmic-clash-server
+WORKDIR /opt/cosmic-clash
+EXPOSE 7777/udp
+ENTRYPOINT ["/opt/cosmic-clash/cosmic-clash-server"]
+
+# Test-only target: runs the source client harness against the exported server.
+FROM exporter AS smoke-client
+WORKDIR /workspace
+ENTRYPOINT ["godot", "--headless", "--path", "Game", "res://tests/export_server_smoke.tscn", "--"]