Adding a pattern
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Spec: descriptive guidance, not a checkable rule.
It carries no forbidden and instead pair, and no `jig check` detector applies to it directly.
Section: Components and layout

A new pattern earns a place here when it has been built three times. Before then it is a component, not a pattern.

Each entry states: anatomy in order, complete state list, rules that are decidable, and mode variance. If a rule cannot be checked by looking at the output, it belongs in `04-principles.md`.