Encodes/decodes symbol streams under PMFs represented as quantized counts out of a caller-supplied total. Correctness over speed. ## Design Uses the LZMA-style carry-propagation range coder: - `range` starts at `0xFFFF_FFFF` (`2^32 - 1`). - `low` is tracked as a 64-bit integer; bit 32 signals a carry. - The encoder emits a leading alignment byte (`0x00`) followed by data bytes. - The decoder preloads 5 bytes at init (one is the alignment byte). - Carry propagation is handled via a single-byte cache plus a pending-`0xFF` count.