Critical fixes from whole-branch code review:
- libaec-sys: fix flag constants to match <libaec.h> exactly
(PREPROCESS=8, MSB=4, RESTRICTED=16; drop non-existent AEC_ALLOW_K13)
and add aec_buffer_encode FFI declaration
- filters_szip: fix cd index for bits_per_sample (cd[2] per H5Z_SZIP_PARM_BPP,
not cd[4]); fix option-mask mapping (NN=0x20, MSB unconditional); add two
real encode→decode roundtrip tests (no-NN and NN) that exercise libaec end-to-end
- file_writer: fix serialize_vds_mappings to delegate to data_layout_write
(eliminates the buggy duplicate that always emitted version=1 even for
external-file mappings); retains trailing Jenkins checksum
Co-Authored-By: Claude Sonnet 4.6 <[email protected]>