more 4.3 features, bug fixes and performance improvements

[ratal]

New Features

• MDF4 writing: CA block, CH block, DS/CL and CV/CU compositions, SR blocks, and Data Stream channels
• Unfinalised MDF4 file reading
• Channel Variants and Union array parsing
• Frequency sync channel type support
• Data stream mode for large files
• Schema-based typed metadata (MdComment) for HD, FH, CN, CG, CC, SI, EV, AT, CH, DG blocks
• Python: PyErr exposed for conversion into Python exceptions

Bug Fixes

• Fixed VLSD reading regression
• Fixed value range conversion not matching MDF spec
• Fixed DL/HL/DZ block handling for VLSC channels
• Fixed UTF-16 wrong endianness
• Fixed half-float (f16) reading
• Fixed critical safety issues in the C API

Performance

• ~28% faster read on simple files
• Optimised performance for files with many channels
• Optimised VLSC sorted/unsorted data parsing
• Optimised CV and CU channel handling

Refactoring

• Split mdfinfo4 into per-block-type sub-modules
• Endianness representation made less ambiguous
• Replaced expect()/unwrap() calls with proper error propagation
• ChannelData::Display made fault-tolerant (logs errors instead of panicking)

Testing & CI

• Added tests for BigEndian channels (via LD blocks), Complex32, and ArrayDFloat64
• Increased overall test coverage
• Switched CI from nightly to stable Rust toolchain
• Added more Python version targets for wheel generation
• Updated all Cargo dependencies