feat(dataflow): govern reusable definition updates

README.md

@@ -137,6 +137,13 @@ records the effective Policy decision and ancestor limits. Inherited
 definitions remain read-only; lower scopes may narrow, but not broaden,
 execution, reuse, inheritance, or automation permissions.
 
+Derived definitions report when their source has a newer immutable revision;
+the source never mutates the child silently. Adopting an update requires the
+reviewed source revision and hash plus a reason. It appends a new child
+revision, retains the previous graph and all run evidence, records reviewer
+and Policy provenance, and returns the child to draft before the changed graph
+can run or receive automation.
+
 Complete active flows support explicit user/API starts, administrative
 backfills, one-time schedules, interval schedules, and exact-match platform
 events. Trigger deliveries are durable and idempotent. They enqueue the same
@@ -147,11 +154,15 @@ the run before source access or output publication.
 Confidential and restricted events are not accepted through the direct
 ingress; those require Core's transactional event bridge.
 
-Reusable subflow nodes pin a template reference, version, graph snapshot, and
-parameter values. Their single input is bound to an explicitly marked inline
-source inside the snapshot, parameter substitution is data-only, and nesting
-is bounded. This keeps completed run definitions reproducible even when the
-source template changes later.
+Reusable subflow nodes select a Policy-authorized complete flow or template and
+an immutable revision. The server resolves the graph instead of accepting a
+caller-supplied snapshot, records the source hash and Policy decision, and pins
+closed typed input/output contracts. Their single input is bound to an
+explicitly marked typed inline source inside the snapshot, parameter
+substitution is data-only, and cycles across nested references are rejected.
+Incompatible caller schemas fail validation before execution. This keeps
+completed run definitions reproducible even when the source definition changes
+later.
 
 The executable fixtures in `fixtures/golden` cover monthly structured-file
 reconciliation, sanctions screening, a HEICO-style current-status export, and