nx_range_coder.nx

nx_range_coder.nx -- arithmetic (range) coder for the Nishi voice codec. Phase 4a building block. Pure integer; tier-0 ready. What it does: takes a stream of symbols, each with a probability distribution, and emits a near-optimal-entropy byte stream. The decoder reverses. This is what makes the codec hit its target bitrate -- LPC + ACE produce VALUES; range-coder packs them into BYTES with near-Shannon-optimal density. Algorithm: standard binary arithmetic coder operating on 32-bit integer range. Range starts at [0, 2^32). Each symbol narrows the range proportional to its cumulative probability. When the top byte is determined we emit it. RFC 6716 (Opus) section 4 has the same shape; we re-derive from the algorithm, not the bytes. State is i64 to give headroom on the multiplications. genealogy_id: rissanen_1976_arithmetic_coding + opus_range_coder_4 + rfc_6716_section_4 lineage_id: nishi_range_coder_q10