ci: remove unsupported upload-artifact@v4 from doc-build job

Gitea Actions on GHES does not support actions/upload-artifact@v4 — the
v4 release switched to GitHub-only APIs (artifact backend rewritten).
The doc-build job was failing with "artifact@v4+ are not currently
supported on GHES."

Changes
───────
* Removed the artifact-upload step entirely.  Rationale: the warning
  summary in the job log is the primary reviewer signal for the
  documentation health check.  Reviewers who want to inspect the HTML
  locally can rebuild it with `cmake --build build --target doc`.
* Removed the apt-get install step.  Doxygen is now pre-installed in
  the ci-cpp container (Dockerfile change earlier in this PR).
* Added an explanatory comment so the missing artifact step is not
  re-introduced unknowingly.
* Added a "Report HTML output" step that prints file count + total size
  for visibility (a no-op if the HTML directory is absent).

When/if a real artifact host appears (Gitea Pages, S3, GitHub mirror
release), this job can be extended to publish the HTML there.  For now,
the in-log warning summary is sufficient.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/doc-build.yaml

@@ -9,12 +9,16 @@ on:
 # ─────────────────────────────────────────────────────────────────────────────
 # Doc-build — informational only
 #
-# Generates Doxygen HTML from the public headers and uploads it as an
-# artifact for reviewer inspection.  Does NOT block merges:
-# `continue-on-error: true` ensures warnings or extraction issues never
-# fail the CI gate.  When Doxygen coverage is denser (Phase 8c), this job
-# can be promoted to a hard requirement and the artifact deployed to
-# Pages.
+# Generates Doxygen HTML from the public headers and reports warning
+# statistics.  Does NOT block merges: `continue-on-error: true` ensures
+# warnings or extraction issues never fail the CI gate.  When Doxygen
+# coverage is denser (Phase 8c), this job can be promoted to a hard
+# requirement and the HTML deployed to Pages.
+#
+# Note: Gitea Actions on GHES does not support `actions/upload-artifact@v4`,
+# so HTML artifact upload is intentionally omitted.  The warning summary
+# in the job log is the primary reviewer signal; reviewers who want the
+# HTML can rebuild it locally with `cmake --build build --target doc`.
 # ─────────────────────────────────────────────────────────────────────────────
 jobs:
   doc-build:
@@ -26,9 +30,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install Doxygen
-        run: apt-get update && apt-get install -y --no-install-recommends doxygen
-
       - name: Generate API documentation
         run: doxygen Doxyfile 2>&1 | tee doxygen.log
 
@@ -38,13 +39,18 @@ jobs:
           if [ -f doc/doxygen/doxygen-warnings.log ]; then
             warn=$(wc -l < doc/doxygen/doxygen-warnings.log)
             echo "DOC ▸ Doxygen warnings: $warn"
+            echo ""
+            echo "First 20 warnings:"
             head -20 doc/doxygen/doxygen-warnings.log
+          else
+            echo "DOC ▸ No warning log produced — check that Doxyfile WARN_LOGFILE points to doc/doxygen/doxygen-warnings.log"
           fi
 
-      - name: Upload HTML as artifact
+      - name: Report HTML output
         if: always()
-        uses: actions/upload-artifact@v4
-        with:
-          name: doxygen-html
-          path: doc/doxygen/html/
-          retention-days: 14
+        run: |
+          if [ -d doc/doxygen/html ]; then
+            files=$(find doc/doxygen/html -type f | wc -l)
+            size=$(du -sh doc/doxygen/html | cut -f1)
+            echo "DOC ▸ HTML output: $files files, $size total"
+          fi