ci: add branch coverage to lcov

Enable branch coverage with --rc lcov_branch_coverage=1.
This provides more detailed coverage metrics:
- Line coverage: % of code lines executed
- Function coverage: % of functions called
- Branch coverage: % of if/else/switch branches taken

Branch coverage is a step towards MC/DC (Modified Condition/Decision Coverage)
used in safety-critical systems.

Also add coverage/ to .gitignore.

.github/workflows/ci-code.yml

@@ -113,10 +113,13 @@ jobs:
             make -j$(nproc)
             make check || true
             make check-functional-parallel || true
-            lcov --capture --directory . --output-file /coverage/coverage.info --ignore-errors inconsistent
+            lcov --capture --directory . --output-file /coverage/coverage.info \
+              --rc lcov_branch_coverage=1 \
+              --ignore-errors inconsistent
             lcov --remove /coverage/coverage.info \
               "/usr/include/*" "*/tests/*" \
               --output-file /coverage/coverage.info \
+              --rc lcov_branch_coverage=1 \
               --ignore-errors inconsistent,empty,unused
           '
       - name: Upload coverage to Codecov