feat: add code coverage with branch coverage

- Add --enable-coverage option to configure.ac
- Add coverage/coverage-html targets to Makefile.am
- Add coverage CI job with lcov + Codecov upload
- Include both unit and functional tests in coverage
- Enable branch coverage (step towards MC/DC)
- Add lcov to all Dockerfiles
- Add coverage/ to .gitignore

Makefile.am

@@ -386,6 +386,20 @@ check-unit: tests/unittests/unittests
 @VALGRIND_CHECK_RULES@
 VALGRIND_SUPPRESSIONS_FILES=prof.supp
 
+# Code coverage targets (requires --enable-coverage)
+coverage-clean:
+	find . -name '*.gcda' -delete
+	find . -name '*.gcno' -delete
+	rm -rf coverage-html coverage.info
+
+coverage-report: check
+	lcov --capture --directory . --output-file coverage.info --ignore-errors inconsistent
+	lcov --remove coverage.info '/usr/*' '*/tests/*' --output-file coverage.info --ignore-errors inconsistent
+	genhtml coverage.info --output-directory coverage-html
+	@echo "Coverage report generated in coverage-html/index.html"
+
+.PHONY: coverage-clean coverage-report
+
 format: $(all_c_sources)
 	clang-format -i $(all_c_sources)