MNF13-R5 promoted: src/mix_coder.rs = per-context FAST (cap 4,096)
and SLOW (cap 65,536) adaptive fixed-point (2^24) estimators mixed by
a log-space EWA weight updated per symbol from the coded symbol —
identical model evolution both sides (zero transport); constriction
0.5's stream range coder carries the probabilities (the rans_mixed
precedent's proven machinery; a new direct dep of the shipped crate).

Wire: backend id 4 = [u32 stream_len] + LE u32 words; validation =
word-alignment + the stream-length bound + truncated stream = Err
never panic. The pick (Maximum) now measures all three backends at
complete serialized cost per arm; Balanced stays zstd-19-pinned.

Gate: make gate OK; corpus (CLI, 30/30 verify PASS) 816,341 B =
−16,398 (−1.97% vs v1.12), mix picks on 20/30 (photographic class
wins −3..−7% each); enc-time +40% (measured, the complete-search
cost); fuzz 1,025 trials 0 panics (4 accepted = diffed benign);
conformance +2 (mix golden LEFT|REF=3 0xe0 + unaligned/bomb
rejects).

Jon <jon@nocturnalinc.com>