rice: extract encode/decode helpers, add test runner + fuzz mli

Split encode_ccsds_block into best_split_k/encode_second_extension/
encode_uncompressed/encode_split. Pull decode_zero_blocks/decode_second
_extension/decode_uncompressed/decode_split out of decode_ccsds_blocks.
Compress now delegates each block to encode_one_block and a
count_zero_run helper; decompress factors out decode_rsi /
decompress_blocks. Move Alcotest.run from test_rice.ml into a new
test.ml exporting the rice suite. Add fuzz_rice.mli interface.