Keep a composer request's seats through the decode, the fill and the marshal (issue 1921)

A build module that fills a request re-marshals it, and without the
field it dropped the seat list the trunk's manifest validator reads, so
the validator never got one (milestone 47/B0, ADR 0636 §4, ADR 0644).

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/composer/composer_test.go

@@ -12,7 +12,9 @@ const demo = `{"module": "demo", "version": "1", "claims": [{"name": "node-files
 // A manifest named by its digest goes on the wire as {"digest": ...}, and Fill puts the bytes the indexer keeps in its
 // place; a raw manifest goes as itself.
 func TestAManifestIsItsBytesOrItsDigest(t *testing.T) {
-	r := Request{V: Version, Cases: []Case{{Name: "change", Manifests: map[string]Manifest{
+	// 1921 (B0): the seat list rides whole through the wire, the decode and the fill.
+	seats := []json.RawMessage{json.RawMessage(`{"name":"node-files","verbs":["read"]}`)}
+	r := Request{V: Version, Seats: seats, Cases: []Case{{Name: "change", Manifests: map[string]Manifest{
 		"demo": {Digest: "d1"}, "raw": {Raw: json.RawMessage(`{"module":"raw","version":"1"}`)}},
 		Kept: map[string]map[string]Manifest{"laptop": {"demo": {Digest: "d1"}}}}}}
 	wire, err := json.Marshal(r)
@@ -36,6 +38,10 @@ func TestAManifestIsItsBytesOrItsDigest(t *testing.T) {
 	}); err != nil {
 		t.Fatal(err)
 	}
+	refilled, err := json.Marshal(back)
+	if err != nil || !strings.Contains(string(refilled), `"seats":[{"name":"node-files","verbs":["read"]}]`) {
+		t.Fatalf("the seats after the fill: %s %v", refilled, err)
+	}
 	if asked != 2 || string(back.Cases[0].Manifests["demo"].Raw) != demo || string(back.Cases[0].Kept["laptop"]["demo"].Raw) != demo {
 		t.Fatalf("filled %d times: %+v", asked, back.Cases[0])
 	}