ui/phase-24: declare game.turn.ready as JSON-friendly catalog kind

TestBuildClientPushEventCoversCatalog required every catalog kind to
encode through a FlatBuffers `preMarshaledEvent`. game.turn.ready
intentionally rides on the JSON fallback because its payload is just
`{game_id, turn}` and the only consumer (Phase 24 UI handler) parses
JSON inline. Make the policy explicit through a jsonFriendlyKinds
allow-list so the test still asserts each kind is covered and a future
producer that picks the wrong encoding fails loudly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/notification/events_test.go

@@ -9,9 +9,25 @@ import (
 	"github.com/google/uuid"
 )
 
+// jsonFriendlyKinds lists catalog kinds whose payload is small and
+// stable enough that the gateway-bound encoding stays JSON instead of
+// FlatBuffers. The default for new producers is still FB; declaring a
+// kind here is a deliberate decision baked into the build target's
+// payload contract.
+//
+// `game.turn.ready` ships `{game_id, turn}` only, the UI parses it
+// inline in `routes/games/[id]/+layout.svelte` (Phase 24), and no
+// other consumer reads the payload — adopting the FB encoder would
+// require a new TS notification stub set and the regen tooling for
+// `pkg/schema/fbs/notification.fbs` without buying anything.
+var jsonFriendlyKinds = map[string]bool{
+	KindGameTurnReady: true,
+}
+
 // TestBuildClientPushEventCoversCatalog asserts that every catalog kind
-// returns a typed FB event (preMarshaledEvent) and that an unknown kind
+// is exercised by this test, that FB-typed kinds return a
-// falls through to the JSON safety net.
+// `preMarshaledEvent`, and that JSON-friendly kinds (see
+// `jsonFriendlyKinds` above) return a `push.JSONEvent`.
 func TestBuildClientPushEventCoversCatalog(t *testing.T) {
 	t.Parallel()
 
@@ -57,6 +73,10 @@ func TestBuildClientPushEventCoversCatalog(t *testing.T) {
 			"game_id": gameID.String(),
 			"reason":  "missing engine version",
 		}},
+		{"game turn ready", KindGameTurnReady, map[string]any{
+			"game_id": gameID.String(),
+			"turn":    int32(7),
+		}},
 	}
 
 	seenKinds := map[string]bool{}
@@ -78,8 +98,10 @@ func TestBuildClientPushEventCoversCatalog(t *testing.T) {
 			if len(bytes) == 0 {
 				t.Fatalf("Marshal returned empty bytes")
 			}
-			if _, isJSON := event.(push.JSONEvent); isJSON {
+			_, isJSON := event.(push.JSONEvent)
-				t.Fatalf("expected typed FB event for %s, got JSONEvent", tt.kind)
+			wantJSON := jsonFriendlyKinds[tt.kind]
+			if isJSON != wantJSON {
+				t.Fatalf("kind %s: JSONEvent=%v, want JSONEvent=%v", tt.kind, isJSON, wantJSON)
 			}
 		})
 		seenKinds[tt.kind] = true