Nineteen files, seventy-nine mentions, no definition. That is how-we-build §5
failing on this repository's own vocabulary — ubiquitous language is checked,
not assumed.
The definition, and it is not arbitrary: the control plane is everything that
needs to know about MORE THAN ONE NODE. It follows from ADR 0037, which has the
host applying rather than deciding precisely because deciding needs knowledge
the machine does not have. So the line falls exactly there — writing a file is
the host's, choosing which nodes run the store is the control plane's, and
anything a single machine could answer alone does not belong here at all.
That last consequence is worth having: putting a single-machine concern in tier
2 is a mistake the tier rule will NOT catch, because the dependency direction
stays correct.
Also states what it is not — not the thing that changes machines, not a surface,
not the substrate, and not privileged on a node beyond what the declaration
vocabulary allows. And the property that makes tier 2 unlike the others: it is
itself a consumer, with the same requirements as any module, which is the
circularity the bundle exists to resolve rather than hide.
Scoped deliberately: this defines the term and does not design the contexts
inside it. Ten is the skeleton's claim rather than a settled list, and research
006 still asks whether the record belongs here or in the substrate.
