add EBCC chunking C API

[huanglangwen]

Summary

• add chunked EBCC encode/decode entry points with a serialized chunking header
• support default full-array chunking, slab reuse, padded edge chunks, and decode fallback for non-chunked EBCC payloads
• extend the Python ctypes config layout and initialize chunk dimensions in the HDF5 config path
• add ctypes C-API tests covering headers, contiguous and non-contiguous chunks, oversize chunk dimensions, and roundtrip behavior
• fix CI checkout so submodules can resolve pinned commits behind annotated tags

Validation

• .venv/bin/python -m pytest tests/test_c_api.py