- Hoist RANS_L to module-level const (was duplicated in encode/decode)
- Add doctest to module doc (README example now compiled)
- Add doc comments to all public accessors and AnsError variants
- Add examples/basic.rs
- Add tests: single-symbol alphabet, empty message, precision
  boundaries (1,2,19,20), error paths (invalid precision, empty
  counts, all-zero counts)
- Widen proptest precision range from 8..14 to 1..15
- Simplify unused destructuring in sym_by_slot loop
- Remove commented-out bench config and unused criterion dep