feat: add Dolphin traversal server for GameCube/Wii netplay

Self-hosted NAT hole-punching relay for Dolphin emulator netplay.
Enables online play for ALL GameCube and Wii games without players
needing to open ports (Mario Party 4-7, MKDD, Smash Melee, etc).

Multi-stage Docker build compiles only the traversal_server target.
UDP ports 6262 (primary) and 6226 (NAT probe).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

servers/dolphin-traversal/.dockerignore

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+*.md
+.git

servers/dolphin-traversal/Dockerfile

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+# Dolphin Emulator Traversal Server
+# Lightweight NAT hole-punching relay for Dolphin netplay
+# Supports ALL GameCube/Wii games via Dolphin
+
+# --- Build stage ---
+FROM debian:bookworm AS build
+
+RUN apt-get update && apt-get install -y \
+    build-essential cmake git pkg-config \
+    libfmt-dev libenet-dev libcurl4-openssl-dev \
+    libbz2-dev liblzma-dev libzstd-dev zlib1g-dev \
+    liblzo2-dev liblz4-dev libspng-dev \
+    libusb-1.0-0-dev libevdev-dev libpugixml-dev libxxhash-dev \
+    libminiupnpc-dev libhidapi-dev libsystemd-dev libudev-dev \
+    glslang-dev glslang-tools \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /dolphin
+
+RUN git clone --depth 1 https://github.com/dolphin-emu/dolphin.git . \
+    && git submodule update --init --recursive --depth 1
+
+RUN mkdir build && cd build && cmake .. \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DENABLE_QT=OFF \
+    -DENABLE_NOGUI=OFF \
+    -DENABLE_CLI=OFF \
+    -DENABLE_TESTS=OFF \
+    -DUSE_DISCORD_PRESENCE=OFF \
+    -DENABLE_AUTOUPDATE=OFF \
+    -DENABLE_ANALYTICS=OFF
+
+RUN cd build && cmake --build . --target traversal_server -j$(nproc)
+
+# --- Runtime stage ---
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y \
+    libfmt9 libstdc++6 \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN useradd -r -s /bin/false dolphin
+
+COPY --from=build /dolphin/build/Binaries/traversal_server /usr/local/bin/traversal_server
+
+USER dolphin
+
+EXPOSE 6262/udp
+EXPOSE 6226/udp
+
+CMD ["traversal_server"]