COMP: Always run the Doxygen check on PRs; cache the build tree

Dropping the pull_request paths filter lets the check become a
required status without stranding filtered PRs. The build-doxygen
cache trims the configure cost, and failures now also upload the
effective Doxyfile alongside the warning log.

Author: hjmjohnson

.github/workflows/doxygen.yml

@@ -1,5 +1,7 @@
 name: ITK.Doxygen.Warnings
 
+# pull_request intentionally has no paths-ignore: the check always runs, so it
+# can safely be made a required status without stranding filtered PRs.
 on:
     push:
         branches:
@@ -13,13 +15,6 @@ on:
             - 'Utilities/ITKv5Preparation/**'
             - 'Utilities/Maintenance/**'
     pull_request:
-        paths-ignore:
-            - '*.md'
-            - LICENSE
-            - NOTICE
-            - 'Utilities/Debugger/**'
-            - 'Utilities/ITKv5Preparation/**'
-            - 'Utilities/Maintenance/**'
 
 concurrency:
   group: '${{ github.workflow }}@${{ github.head_ref || github.ref }}'
@@ -40,13 +35,23 @@ jobs:
       - name: Set up Pixi
         uses: prefix-dev/setup-pixi@v0.9.5
 
+      - name: Restore Doxygen build tree
+        uses: actions/cache@v5
+        with:
+          path: build-doxygen
+          key: doxygen-build-${{ hashFiles('CMakeLists.txt', 'CMake/**', 'Utilities/Doxygen/**', 'pyproject.toml', 'pixi.lock') }}
+          restore-keys: |
+            doxygen-build-
+
       - name: Run warnings-only Doxygen pass
         run: pixi run doxygen-ci
 
-      - name: Upload warnings log
+      - name: Upload warnings log and effective configuration
         if: failure()
         uses: actions/upload-artifact@v4
         with:
           name: doxygen-warnings
-          path: build-doxygen/doxygen-warnings.log
+          path: |
+            build-doxygen/doxygen-warnings.log
+            build-doxygen/Utilities/Doxygen/Doxyfile.Documentation
           if-no-files-found: ignore