Write the mission-data boundary brief

[rusholme]

What the purpose requires

The boundary between mission-supplied data and RAPID-derived products
needs to be written down as a design statement, so downstream
decisions (retention, provenance, reprocessing) can be made against a
stated boundary rather than an implicit one.

What exists today

The boundary between mission-supplied data and RAPID-derived products
is not written down as a design statement anywhere.

The trigger that reopens this

The next design or provenance question that depends on where this
boundary falls and cannot be answered from an existing document, or
before the next design-authority (rapid_docs) review pass.

[rusholme]

Triaged 2026-09-21 against the RAPID rebuild specification (https://roman-rapid.readthedocs.io/en/latest/system/specification.html). The rebuild on SMDC starts from a fresh sheet: existing infrastructure, code and database contents are disposable and are recreated to that specification, so issues about the accumulated system do not carry forward as tracked items. The requirement this issue states was carried into the specification (Constraints or Not decided here, rapid_docs pull request #3). Closing; the specification is now where it is tracked.