Architecture decision reference

Architecture decision records capture accepted cross-repository contracts before package implementations diverge. They are coordination records, not runtime dependencies.

• The ADR index records status and supersession rules.
• ADR 0001 defines the accepted multi-response observation and simulator-result contract.

An ADR's review occurs in its pull request. A later decision supersedes an ADR by linking back to it; editing history in place should not obscure the original decision.