ENH: Add Qt6 build to CI workflow

- Extend matrix with qt-major-version (5, 6) to exercise both Qt versions.
- Split Qt install into Qt 5 and Qt 6 steps conditioned on qt-major-version.
- Keep using system VTK only for Qt 5 builds; disable it for Qt 6 builds.
- Derive CTK_QT_VERSION from qt-major-version in the configure step.
- Update diagnostic step to print both Qt5_DIR and Qt6_DIR from CMake cache.

Author: jcfr

.github/workflows/ci.yml

@@ -21,6 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         runs-on: ["ubuntu-latest"]
+        qt-major-version: [5, 6]
     steps:
       - uses: actions/checkout@9bb56186c3b09b4f86b1c65136769dd318469633 # v4.1.2
 
@@ -36,7 +37,8 @@ jobs:
       - name: Retrieve new lists of APT packages
         run: sudo apt-get update
 
-      - name: Install Qt
+      - name: Install Qt 5
+        if: ${{ matrix.qt-major-version == 5 }}
         run: |
           sudo apt-get install -y \
             qtbase5-dev \
@@ -50,17 +52,33 @@ jobs:
             libqt5x11extras5-dev \
             libxt-dev
 
-      - name: Install VTK
-        if: matrix.runs-on == 'ubuntu-latest'
+      - name: Install VTK (Qt 5)
+        if: ${{ matrix.runs-on == 'ubuntu-latest' && matrix.qt-major-version == 5 }}
         run: |
           sudo apt-get install -y \
             libvtk9-dev \
             libvtk9-qt-dev
 
+      - name: Install Qt 6
+        if: ${{ matrix.qt-major-version == 6 }}
+        run: |
+          sudo apt-get install -y \
+            qt6-base-dev \
+            qt6-base-private-dev \
+            qt6-5compat-dev \
+            qt6-multimedia-dev \
+            qt6-scxml-dev \
+            qt6-tools-dev \
+            qt6-tools-dev-tools \
+            qt6-svg-dev \
+            qt6-webengine-dev \
+            libqt6openglwidgets6 \
+            libxt-dev
+
       - name: Configure CTK
         run: |
           cmake \
-            -DCTK_QT_VERSION:STRING=5 \
+            -DCTK_QT_VERSION:STRING=${{ matrix.qt-major-version }} \
             -DCTK_ENABLE_Widgets:BOOL=ON \
             -DCTK_USE_SYSTEM_VTK:BOOL=$CTK_USE_VTK \
             -DCTK_LIB_Visualization/VTK/Widgets:BOOL=$CTK_USE_VTK \
@@ -81,11 +99,14 @@ jobs:
             -B CTK-build \
             -S .
         env:
-          CTK_USE_VTK: "${{ matrix.runs-on == 'ubuntu-latest' && 'ON' || 'OFF' }}"
+          CTK_USE_VTK: "${{ matrix.runs-on == 'ubuntu-latest' && matrix.qt-major-version == 5 && 'ON' || 'OFF' }}"
 
-      - name: Display Qt5_DIR
+      - name: Display Qt_DIR
         run: |
-          cat CTK-build/CMakeCache.txt | grep ^Qt5_DIR
+          echo "Qt5_DIR:"
+          cat CTK-build/CMakeCache.txt | grep '^Qt5_DIR' || true
+          echo "Qt6_DIR:"
+          cat CTK-build/CMakeCache.txt | grep '^Qt6_DIR' || true
 
       - name: Build CTK
         run: |