Rename spo2_algorithm target to spo2_algorithm-lib, use FetchDependencies, gate demo executable
- Rename library target from "spo2_algorithm" to "spo2_algorithm-lib"
- Replace manual FetchContent blocks for averaging, c_helpers, wfdb_process, csv_process, and filtering with the new FetchDependencies module and fetchdependencies()
- Add LIB_DEMO_ENABLE option (default OFF) to conditionally build the demo executable
- Drop -Wall -Wextra -Werror compile options and redundant enable_language(C CXX) call
- Rename DATASET_DIR to SPO2_ALGORITHM_DATASET_DIR
- Update project.yml.in to reference "-lib" suffixed static library filenames, and CI CMAKE_ARGS to pass -DLIB_DEMO_ENABLE=ON
- Document the LIB_DEMO_ENABLE option in the README and update the doc-generation command