A build records the bases it was handed, and the mesh reads its edges from builds

Bases reach a recipe as build arguments, so the digest was never in the file the builder read
edges from: no build on the mesh recorded what it stood on, and 'build --on', the bases-first
order and the merge follow-up all walked a graph with no edges (novox/hq 04-ISSUES/131). The
builder now reports every base it resolved; the controller records them by artifact path and
reads the newest build's edges from the store, since a recorded manifest carries no build.on.

internal/inventory/builds.go

@@ -164,6 +164,41 @@ func (i *Inventory) Held(ctx context.Context) (map[string]string, error) {
 	return held, rows.Err()
 }
 
+// BuiltAgainst is what each module's newest successful build stood on, as recorded — the build
+// edges (ADR 0009). A module whose last build recorded no bases is absent, which is also what a
+// module standing on nothing looks like: an edge the mesh has not derived is not an edge.
+func (i *Inventory) BuiltAgainst(ctx context.Context) (map[string][]string, error) {
+	rows, err := i.store.Pool().Query(ctx,
+		`select distinct on (module) module, built_against
+		   from build
+		  where module is not null and module <> '' and failed = ''
+		  order by module, at desc`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	against := map[string][]string{}
+	for rows.Next() {
+		var module string
+		var raw []byte
+		if err := rows.Scan(&module, &raw); err != nil {
+			return nil, err
+		}
+		if len(raw) == 0 {
+			continue
+		}
+		var refs []string
+		if err := json.Unmarshal(raw, &refs); err != nil {
+			continue
+		}
+		if len(refs) > 0 {
+			against[module] = refs
+		}
+	}
+	return against, rows.Err()
+}
+
 // manifestOrNil keeps the difference between "declared nothing" and "predates this being kept".
 //
 // A build recorded before the mesh kept manifests has no manifest, and that is not the same as one