piyxu/256bits-to-128k-rank-5bit-x21--metadata: 256-bit to 128k Balanced Rank Encoding with 5-bit x21 Metadata

This release introduces a simple reversible encoding system that maps any 256-bit value into a 256-bit balanced binary block with exactly 128 ones, plus a small metadata value. The system is based on the combinational space C(256,128). Since C(256,128) covers about 4.9819% of the full 256-bit numeric space, the full 256-bit range is divided into 21 metadata pools. Each pool points to one C(256,128)-sized balanced block space. In the basic version, each encoded block uses: 5-bit metadata + 256-bit balanced block