build: add ci workflow for cygwin build and test

This patch introduces a workflow that verifies argtable3 can be built
and tested using Cygwin on Windows. Both Debug and Release
configurations are built and tested. The workflow ensures Cygwin
compatibility and test coverage on every change.

Author: tomghuang

.github/workflows/cygwin.yml

@@ -0,0 +1,57 @@
+name: Cygwin Build
+
+on:
+  workflow_dispatch:
+  push:
+    branches: [ master ]
+  pull_request:
+
+jobs:
+  build:
+    name: Build with Cygwin (${{ matrix.configuration }})
+    runs-on: windows-latest
+    strategy:
+      matrix:
+        configuration: [Release, Debug]
+
+    steps:
+      - name: Install Cygwin and dependencies
+        uses: cygwin/cygwin-install-action@v4
+        with:
+          packages: >-
+            gcc-core
+            gcc-g++
+            make
+            cmake
+
+      - name: Add Cygwin to PATH
+        run: echo "C:\\cygwin64\\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+        shell: pwsh
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Get Cygwin path for workspace
+        id: cygpath
+        shell: C:\cygwin64\bin\bash.exe --login -eo pipefail {0}
+        run: |
+          echo "cygwin_path=$(cygpath -u "$GITHUB_WORKSPACE")" >> $GITHUB_ENV
+
+      - name: Configure with CMake (Cygwin)
+        shell: C:\cygwin64\bin\bash.exe --login -eo pipefail {0}
+        run: |
+          cd "$cygwin_path"
+          cmake -B build/${{ matrix.configuration }} \
+            -G "Unix Makefiles" \
+            -DCMAKE_BUILD_TYPE=${{ matrix.configuration }}
+
+      - name: Build with CMake (Cygwin)
+        shell: C:\cygwin64\bin\bash.exe --login -eo pipefail {0}
+        run: |
+          cd "$cygwin_path"
+          cmake --build build/${{ matrix.configuration }} -- -j
+
+      - name: Run tests (fail if any test fails)
+        shell: C:\cygwin64\bin\bash.exe --login -eo pipefail {0}
+        working-directory: ${{ env.cygwin_path }}/build/${{ matrix.configuration }}
+        run: ctest --output-on-failure