Schedule mutation testing outside push CI

Move the full mutation suite into a dedicated daily 02:00 EST workflow so merge and release checks do not consume CI agents for hours.

.gitea/workflows/mutation.yaml

@@ -0,0 +1,48 @@
+name: akbasic Mutation Testing
+run-name: ${{ gitea.actor }} akbasic mutation test
+
+# Mutation testing is intentionally isolated from push and merge checks. The
+# complete src/ tree takes hours and must not consume the shared CI capacity
+# during normal development.
+on:
+  # Gitea schedules use UTC. 07:00 UTC is 02:00 EST (the requested fixed
+  # Eastern Standard Time slot).
+  schedule:
+    - cron: '0 7 * * *'
+  workflow_dispatch:
+
+jobs:
+  full_mutation:
+    runs-on: ubuntu-latest
+    timeout-minutes: 720
+    steps:
+      - name: Check out repository code
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+      - name: dependencies
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y cmake gcc python3 moreutils
+      - name: mutation testing (full tree)
+        run: |
+          python3 scripts/mutation_test.py \
+            --junit mutation-junit.xml \
+            --threshold 65
+      - name: publish mutation results
+        if: always()
+        uses: mikepenz/action-junit-report@v4
+        with:
+          report_paths: 'mutation-junit.xml'
+          annotate_only: true
+          detailed_summary: true
+          include_passed: true
+          fail_on_failure: 'false'
+      - name: upload mutation report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutation-report
+          path: mutation-junit.xml
+          if-no-files-found: warn
+      - run: echo "🍏 This job's status is ${{ job.status }}."