refactor(wfdb_writer): simplify writer API and support mixed channel frame writing
- Refactor wfdb_writer_init_siginfo() interface and formatting
- Add channel type support (integer/float) to writer context
- Introduce WFDB_ChannelType and WFDB_ChannelValue abstractions
- Simplify wfdb_writer_put_frame() to write internal sample buffer
- Consolidate frame writing into wfdb_writer_write_frame()
- Support mixed integer and floating-point channels in a single frame
- Remove deprecated sample/frame write APIs and unused writer functions
- Remove obsolete enums, declarations, and commented-out legacy code
- Reorganize header declarations for a cleaner public API
- Improve code formatting and internal resource handling