Create domain/app error mapping table for future API layer

[fleuronvilik]

Objective

Prepare Phase 3 by defining boundary-level mapping for typed errors.

Scope

• Map domain and app errors to transport-safe response semantics.
• Add helper for mapping and tests.

Acceptance Criteria

• Mapping table is explicit and documented.
• Helper behavior is covered by unit tests.
• No domain layer depends on HTTP concepts.

Why

A clear error contract is essential before exposing use cases via an API.