Fix #615: Switch from Travis CI to GitHub Actions

Add a GitHub Actions CI workflow with a full build matrix covering
linux, macOS, and Windows with both GCC and Clang compilers, Debug
and Release build types, and optional ASan+UBSan sanitizers (24 builds
total). Windows builds use MSYS2 for GCC/Clang toolchains. The workflow
relies on CMake and CTest to drive configuration, building, and testing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,79 @@
+name: CI
+
+on:
+  push:
+    branches: [develop, main]
+  pull_request:
+    branches: [develop, main]
+
+jobs:
+  build:
+    name: >-
+      ${{ matrix.os }}
+      ${{ matrix.compiler }}
+      ${{ matrix.build_type }}
+      ${{ matrix.sanitizer == 'ON' && 'asan+ubsan' || 'no-sanitizers' }}
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        compiler: [gcc, clang]
+        build_type: [Debug, Release]
+        sanitizer: [ON, OFF]
+        include:
+          - os: windows-latest
+            shell: msys2 {0}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up MSYS2
+        if: runner.os == 'Windows'
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: UCRT64
+          update: false
+          install: >-
+            mingw-w64-ucrt-x86_64-gcc
+            mingw-w64-ucrt-x86_64-clang
+            mingw-w64-ucrt-x86_64-cmake
+            mingw-w64-ucrt-x86_64-ninja
+
+      - name: Install GCC (macOS)
+        if: runner.os == 'macOS' && matrix.compiler == 'gcc'
+        run: brew install gcc
+
+      - name: Set up compiler
+        shell: ${{ matrix.shell || 'bash' }}
+        run: |
+          if [[ "${{ matrix.compiler }}" == "gcc" ]]; then
+            if [[ "${{ runner.os }}" == "macOS" ]]; then
+              GCC_VER=$(brew list --versions gcc | awk '{print $2}' | cut -d. -f1)
+              echo "CC=gcc-${GCC_VER}" >> "$GITHUB_ENV"
+              echo "CXX=g++-${GCC_VER}" >> "$GITHUB_ENV"
+            else
+              echo "CC=gcc" >> "$GITHUB_ENV"
+              echo "CXX=g++" >> "$GITHUB_ENV"
+            fi
+          else
+            echo "CC=clang" >> "$GITHUB_ENV"
+            echo "CXX=clang++" >> "$GITHUB_ENV"
+          fi
+
+      - name: Configure
+        shell: ${{ matrix.shell || 'bash' }}
+        run: >-
+          cmake -B build -G Ninja
+          -DCMAKE_BUILD_TYPE=${{ matrix.build_type }}
+          -DENABLE_ADDRESS_SANITIZER=${{ matrix.sanitizer }}
+          -DENABLE_UNDEFINED_SANITIZER=${{ matrix.sanitizer }}
+
+      - name: Build
+        shell: ${{ matrix.shell || 'bash' }}
+        run: cmake --build build -j
+
+      - name: Test
+        shell: ${{ matrix.shell || 'bash' }}
+        run: ctest --test-dir build --output-on-failure