ci: TEMP native Linux build validation

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: NonPolynomialTim

.github/workflows/ci-linux-validate.yml

@@ -0,0 +1,33 @@
+name: CI linux validate (temp)
+
+# TEMP: prove the coop code builds natively on Linux (GCC) before folding a
+# build-linux job into ci-main. Remove this file once validated.
+on:
+  push:
+    branches: [ci/linux-build]
+  workflow_dispatch:
+
+jobs:
+  build-linux:
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - name: Install deps
+        run: |
+          set -euxo pipefail
+          sudo apt-get update
+          sudo apt-get install -y \
+            cmake g++ make zip \
+            libsdl1.2-dev libsdl-mixer1.2-dev libsdl-gfx1.2-dev libsdl-image1.2-dev libsdl-net1.2-dev \
+            libyaml-cpp-dev libjsoncpp-dev libsodium-dev zlib1g-dev
+
+      - uses: actions/checkout@v4
+        with: { fetch-depth: 0, submodules: recursive }
+
+      - name: Configure + build (Linux x64)
+        run: |
+          set -euxo pipefail
+          rm -rf build && mkdir -p build && cd build
+          cmake -DCMAKE_BUILD_TYPE=Release -DDEV_BUILD=OFF -DBUILD_PACKAGE=OFF "$GITHUB_WORKSPACE"
+          make -j"$(nproc)"
+          ls -la bin/