Breaking changes:
- AnsError::InvalidState fields widened from u32 to u64
  (no longer truncates 64-bit state in error messages)
- Removed empty `std` feature (was a no-op since thiserror removal)

Internal:
- Extract build_lookups() to deduplicate CDF+slot construction
  between from_counts and from_normalized
- Remove redundant "not tuned for speed" note (covered by "Not this" section)
- Display InvalidState in hex for readability