truefix-core — FIX message model, field types, and the SOH wire codec.
Provides [Field], [FieldMap], [Group], and [Message] (header/body/trailer),
plus [encode]/[decode] that compute and verify BodyLength (tag 9) and CheckSum
(tag 10). All parsing returns typed errors and never panics (Constitution Principle I).
Repeating-group structure is modelled here; dictionary-driven group parsing from a
flat wire message arrives with truefix-dict (Stage S4). At this layer, decoding yields
flat ordered fields, which still round-trips byte-for-byte.
Design: specs/001-fix-engine-parity/.