Boundary discipline

Check data where it enters the system and trust it after that.

• Validation, narrowing and error handling belong where data crosses a system boundary: CLI arguments, configuration, the network, an external API.
• At that boundary, a parse function turns raw input into typed domain values.
• Inside, trust data a boundary has validated and propagate errors rather than re-checking it in every layer. Add an inner check only on a failure path a defect has proven, as the bug-fix playbook's hardening step does.
• A public surface exposes domain concepts, never the transport, storage or framework types behind it.
• Business logic lives in pure functions with no framework dependencies; the shell around them stays thin and mechanical and only calls them.