Semantic types for the LimniFS wire format.
Source of truth: limnifs/spec §2.2 (Terminology / semantic types).
Each type is emitted as a distinct newtype (not a bare alias) so the
per-field semantic constraints from the spec (§1.1 multihash display,
§1.4 determinism, exact widths) are enforced at module boundaries.
All widths are exact. Storage MUST NOT widen; width changes are spec amendments, not implementation choices.