ci: speed up builds 4x with parallel tests, coverage, and ccache

Split functional tests into 4 parallel groups and add check-functional-parallel target (~3x faster CI runs).
Add branch-aware LCOV coverage reporting with new --enable-coverage option and lcov summary in CI pipeline.
Enable ccache via -C configure flag for faster recompilations.
Install lcov in all Docker images and use --depth 1 git clones + parallel make -j$(nproc) for quicker container builds.
Update CONTRIBUTING.md with instructions for parallel test groups and adding new ones.

All changes are tightly related CI/performance improvements developed in sequence. No external service uploads (e.g. Codecov skipped due to Gitea incompatibility).

.github/workflows/ci-code.yml

@@ -98,3 +98,28 @@ jobs:
       - name: Check spelling
         run: |
           codespell
+
+  coverage:
+    runs-on: ubuntu-latest
+    name: Code Coverage
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build and run coverage
+        run: |
+          docker build -f Dockerfile.debian -t profanity-cov .
+          docker run -v ${{ github.workspace }}/coverage:/coverage profanity-cov bash -c '
+            ./bootstrap.sh
+            ./configure --enable-coverage --enable-otr --enable-pgp --enable-omemo --enable-plugins
+            make -j$(nproc)
+            make check || true
+            make check-functional-parallel || true
+            lcov --capture --directory . --output-file /coverage/coverage.info \
+              --rc branch_coverage=1 \
+              --ignore-errors inconsistent
+            lcov --remove /coverage/coverage.info \
+              "/usr/include/*" "*/tests/*" \
+              --output-file /coverage/coverage.info \
+              --rc branch_coverage=1 \
+              --ignore-errors inconsistent,empty,unused
+            lcov --summary /coverage/coverage.info
+          '