refactor(wfdb_writer): improve API consistency and C++ compatibility

• Add extern "C" guards for C++ interoperability
• Replace integer open state with bool is_open
• Rename internal members and parameters using p_ prefix for consistency
• Refactor writer API to use updated pointer naming conventions
• Reuse wfdb_writer_close() in error paths
• Add additional input validation for writer APIs
• Close WFDB signal files during writer cleanup