Derive data protection from a module's declared data (hq ADR 0233)

A module's data section says what it keeps and how precious it is; the backup holder's lines,
binding stickiness, retirement on unassign and D13's conditions follow from it, so issue 273's
empty replacement is said and an unassigned module's data is remembered, not forgotten.

cmd/mesh-controller/check.go

@@ -93,6 +93,15 @@ func moduleCheckFor(paths []string, longestMachine int, out io.Writer) error {
 	}
 	failed += len(identities)
 
+	// And the data each module keeps (novox/hq ADR 0233): a provider that grants says what it keeps for
+	// its consumers, a directory a container writes is declared, and no backup line is written by hand.
+	data := catalogue.DataProblems(shelf)
+	sort.Strings(data)
+	for _, p := range data {
+		fmt.Fprintln(out, p)
+	}
+	failed += len(data)
+
 	var names []string
 	for name := range shelf {
 		names = append(names, name)
@@ -121,6 +130,14 @@ func moduleCheckFor(paths []string, longestMachine int, out io.Writer) error {
 		if len(m.Reads) > 0 {
 			fmt.Fprintf(out, ", reads %s", strings.Join(m.Reads, ", "))
 		}
+		// The data it keeps, by class, so a reviewer sees what the mesh will protect and how.
+		if items := m.DataItems(); len(items) > 0 {
+			kept := make([]string, 0, len(items))
+			for _, it := range items {
+				kept = append(kept, it.ID+" ("+it.Class+")")
+			}
+			fmt.Fprintf(out, ", keeps %s", strings.Join(kept, ", "))
+		}
 		fmt.Fprintln(out)
 	}
 	if failed > 0 {